Chemistry Notes – Grade 10
National Curriculum Pakistan - NCP
Chapter 3: Stoichiometry
Aligned with National Curriculum Pakistan (Federal Board, NBF, PTB)
Stoichoimetery Questions and Answers
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What is stoichiometry?
Stoichiometry is the study of quantitative relationships between the reactants and products in a chemical reaction. It helps determine how much of each substance is needed or produced using balanced chemical equations. -
From which Greek words does the term "stoichiometry" come?
The word "stoichiometry" comes from the Greek words “Stoicheion” meaning element and “Metron” meaning measure. It refers to measuring elements in a reaction. -
What is a mole in chemistry?
A mole is the SI unit used to express the amount of a substance. It represents the quantity that contains Avogadro's number (6.022 × 10²³) of particles like atoms or molecules. -
What is the mass of one mole of oxygen gas (O₂)?
The mass of one mole of oxygen gas (O₂) is 32 grams. This is calculated from the atomic mass of oxygen, which is 16, so 16 × 2 = 32 g/mol. -
What is the mass of one mole of Na⁺ (ion)?
The mass of one mole of sodium ion (Na⁺) is 23 grams. This is the atomic mass of sodium since it is a single ion. -
What is the mass of one mole of H₂O?
The molar mass of water (H₂O) is 18 grams. This is calculated by adding the masses of 2 hydrogen atoms (2 × 1) and one oxygen atom (16). -
What is the mole ratio of H₂ and O₂ in the reaction 2H₂ + O₂ → 2H₂O?
In this balanced equation, 2 moles of hydrogen react with 1 mole of oxygen. This shows a 2:1 mole ratio between H₂ and O₂. -
What is the mole ratio of CH₃OH and O₂ in the reaction 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O?
The balanced equation shows that 2 moles of methanol (CH₃OH) react with 3 moles of oxygen (O₂). Therefore, the mole ratio is 2:3. -
How many moles of oxygen are used when 3.5 moles of methanol are burnt?
According to the equation, 2 moles of CH₃OH require 3 moles of O₂. So 3.5 moles of CH₃OH need 5.25 moles of O₂. -
How many moles of water are produced when 3.5 moles of methanol are burnt?
From the balanced equation, 2 moles of CH₃OH give 4 moles of water. So 3.5 moles will produce 7.0 moles of water. -
What is the mass of iron produced from 425 g of Fe₂O₃?
Mass = moles × molar mass = 5.32 × 55.9 = 297.38 g.
So, 297.38 grams of iron can be obtained from 425 g of Fe₂O₃. -
Why is a balanced chemical equation important in stoichiometric calculations?
A balanced equation ensures the law of conservation of mass is followed and gives the correct mole ratio of reactants and products, which is essential for calculating quantities involved in a reaction. -
How does the mole concept simplify chemical calculations?
The mole allows chemists to count particles by weighing them, making it easy to relate mass, volume, and number of particles using a single unit. -
What is the significance of using mole ratios in chemical reactions?
Mole ratios tell us how much of each substance reacts or is produced. They are key to converting between amounts of different substances in a reaction. -
How can you determine the amount of product formed from a given amount of reactant?
Use the balanced equation to find the mole ratio, convert the given mass to moles, use the ratio to find moles of the product, then convert it back to mass if needed. -
What is the mass of iron produced from 425 g of Fe₂O₃?
Mass = moles × molar mass = 5.32 × 55.9 = 297.38 g.
So, 297.38 grams of iron can be obtained from 425 g of Fe₂O₃. -
Why is a balanced chemical equation important in stoichiometric calculations?
A balanced equation ensures the law of conservation of mass is followed and gives the correct mole ratio of reactants and products, which is essential for calculating quantities involved in a reaction. -
How does the mole concept simplify chemical calculations?
The mole allows chemists to count particles by weighing them, making it easy to relate mass, volume, and number of particles using a single unit. -
What is the significance of using mole ratios in chemical reactions?
Mole ratios tell us how much of each substance reacts or is produced. They are key to converting between amounts of different substances in a reaction. -
How can you determine the amount of product formed from a given amount of reactant?
Use the balanced equation to find the mole ratio, convert the given mass to moles, use the ratio to find moles of the product, then convert it back to mass if needed. -
Why is Avogadro’s number important in stoichiometry?
Avogadro’s number (6.022 × 10²³) connects the microscopic scale (atoms/molecules) to the macroscopic scale (grams), allowing practical measurements in the lab. -
Why does burning 3.5 moles of methanol require 5.25 moles of oxygen?
The balanced equation shows that 2 moles of methanol require 3 moles of oxygen. Using this 2:3 ratio, 3.5 moles of methanol need 5.25 moles of O₂ to fully react. -
How does the concept of limiting reactant relate to stoichiometry?
Stoichiometry helps identify the limiting reactant—the one that runs out first—by comparing available moles to the required mole ratios from the balanced equation. -
If the mass of a substance is known, how can you find the number of moles?
Use the formula: Moles = Mass / Molar Mass. This allows you to convert from grams to moles, which are needed for stoichiometric calculations. -
In the reaction Fe₂O₃ + 3CO → 2Fe + 3CO₂, what does the 3 in 3CO indicate?
It indicates that 3 moles of carbon monoxide are required to fully react with 1 mole of Fe₂O₃, based on the mole ratio in the balanced chemical equation. -
Why is the mole used as a standard unit in chemistry instead of counting particles directly?
Because atoms and molecules are extremely small and numerous, it is impractical to count them directly. The mole provides a manageable way to measure quantities. -
If 5 moles of methanol (CH₃OH) are burned, how many moles of oxygen (O₂) are required?
From the balanced equation 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O, the ratio is 2:3.
So, 5 moles CH₃OH require 7.5 moles of O₂. -
How many grams of water are produced when 4 moles of methanol are burned?
From the equation, 2 moles CH₃OH produce 4 moles H₂O. So, 4 moles CH₃OH give 8 moles of H₂O.
Mass = 8 × 18 = 144 grams of water. -
How many moles of hydrogen are needed to produce 6 moles of NH₃?
From the equation N₂ + 3H₂ → 2NH₃, 2 moles NH₃ require 3 moles H₂.
So, 6 moles NH₃ need 9 moles of hydrogen. -
Find the mass of iron produced from 320 g of Fe₂O₃.
Moles of Fe₂O₃ = 320 / 159.6 ≈ 2.004 moles
Since 1 mole of Fe₂O₃ gives 2 moles of Fe: 2.004 × 2 = 4.008 moles of Fe
Mass of Fe = 4.008 × 55.9 = 224 g of Fe -
How many grams of oxygen (O₂) are required to completely react with 10 moles of CH₃OH?
From the equation 2CH₃OH + 3O₂ → products, 10 moles CH₃OH need 15 moles of O₂
Mass = 15 × 32 = 480 g of O₂ -
What is the balanced equation for the reaction of hydrogen with oxygen?
The balanced chemical equation is:
2H₂ + O₂ → 2H₂O -
How many grams of oxygen are required to burn 100 g of hydrogen completely?
From the equation, 2 g of H₂ reacts with 16 g of O₂.
So, for 100 g H₂: (16/2) × 100 = 800 g of oxygen are needed. -
What is the balanced equation for the decomposition of potassium chlorate?
The balanced chemical equation is:
2KClO₃ → 2KCl + 3O₂ -
What is molar volume at RTP and what is its value?
Molar volume is the volume occupied by 1 mole of any gas at room temperature and pressure (RTP).
At RTP (25°C, 1 atm), it is 24 dm³/mol. -
What volume does 1 mole of gas occupy at RTP?
1 mole of any gas occupies 24 dm³ at RTP (room temperature and pressure). -
What volume is occupied by 2.5 moles of chlorine gas at RTP?
Volume = moles × molar volume = 2.5 × 24 = 60 dm³ -
How many moles of oxygen molecules are in 50.0 dm³ of O₂ gas at RTP?
Moles = Volume / Molar volume = 50 / 24 = 2.08 moles -
What volume does 0.80 moles of nitrogen gas occupy at RTP?
Volume = moles × molar volume = 0.80 × 24 = 19.2 dm³ -
What is percentage composition in chemistry?
Percentage composition shows the mass percentage of each element in a compound, based on its molar mass. -
How is percentage composition calculated?
Using the formula:
% of element = (mass of element in 1 mole of compound ÷ molar mass of compound) × 100 -
What is the molar mass of MgO?
Molar mass of MgO = 24 (Mg) + 16 (O) = 40 g/mol. -
What is the percentage of magnesium in MgO?
% of Mg = (24/40) × 100 = 60.3% -
What is the percentage of oxygen in MgO?
% of O = (16/40) × 100 = 39.7% -
What is a limiting reactant?
A limiting reactant is the one that is completely used up in a reaction and limits the amount of product formed. -
What is a non-limiting reactant?
A non-limiting reactant is the one that remains unreacted after the reaction is complete. -
In the reaction 2H₂ + O₂ → 2H₂O, which is the limiting reactant when 10 moles of H₂ and 10 moles of O₂ are used?
H₂ is the limiting reactant because it forms fewer moles of H₂O (10 moles) than O₂ would (which could form 20 moles). -
Why does 1 mole of any gas occupy 24 dm³ at RTP?
At room temperature and pressure (25°C, 1 atm), gases behave ideally and occupy the same volume per mole. This standard molar volume helps compare gases easily in reactions. -
What is the relationship between gas volume and number of moles at RTP?
Gas volume is directly proportional to the number of moles at RTP. Doubling the moles doubles the volume since 1 mole occupies 24 dm³. -
Why do we use percentage composition in chemistry?
Percentage composition helps determine how much of each element is present in a compound. It's useful in chemical analysis and verifying purity or formulas. -
How does knowing molar mass help calculate percentage composition?
The molar mass allows us to find what fraction each element contributes to the total mass of the compound, which we convert into a percentage. -
What does it mean if a substance is a limiting reactant?
It means that the substance is completely used up first in a reaction, stopping further product formation and determining the maximum yield. -
Why is it important to identify the limiting reactant in a reaction?
Identifying the limiting reactant helps predict the exact amount of product formed and prevents wastage of excess reactants. -
In a reaction, what happens to the non-limiting (excess) reactant?
The non-limiting reactant is not completely consumed. Some of it remains unreacted because the limiting reactant runs out first. -
How can you use a balanced equation to identify the limiting reactant?
By calculating how many moles of product each reactant can form based on mole ratios in the balanced equation, the one forming less product is the limiting reactant. -
What is the difference between empirical and molecular formulas?
The empirical formula shows the simplest whole-number ratio of atoms in a compound (e.g., CH₂O).
The molecular formula shows the actual number of atoms of each element in one molecule (e.g., C₆H₁₂O₆).
The molecular formula is always a multiple of the empirical formula.
Empirical formulas are useful in basic analysis, while molecular formulas show complete molecular structure. -
Why do we calculate percentage purity in chemistry?
Percentage purity helps determine how much of a sample is actually the desired pure substance.
Impurities can affect chemical reactions, measurements, and product quality.
This concept is especially important in industries like pharmaceuticals and metallurgy.
A high percentage purity indicates a more accurate and efficient chemical sample for use. -
Why is actual yield often less than theoretical yield in a reaction?
In real-life conditions, reactions may not go to completion.
Losses can occur due to side reactions, evaporation, or incomplete conversion.
Mechanical losses during transfer, filtration, or crystallization are also common.
So, the actual yield is usually lower than the theoretical yield predicted from stoichiometry. -
How do chemists determine an empirical formula from percentage composition?
Chemists first convert percentage composition into moles of each element.
Then, they divide all mole values by the smallest value among them.
This gives a whole-number ratio of atoms in the compound.
These ratios are used as subscripts in writing the empirical formula. -
What is the significance of molecular formula in compound identification?
The molecular formula reveals the exact number of each atom in a molecule.
It allows chemists to identify and distinguish between compounds with the same empirical formula.
It is essential in organic chemistry, drug formulation, and biochemical studies.
For example, C₂H₄O and CH₂O can both be empirical formulas, but their molecular structures are very different. -
What is molarity?
Molarity is the number of moles of solute dissolved per dm³ (liter) of solution.
It is a measure of concentration and is expressed in mol/dm³ or simply M. -
What is the formula for molarity?
Molarity (M) = Moles of solute / Volume of solution in dm³.
You must convert cm³ to dm³ by dividing by 1000. -
What does 18M H₂SO₄ mean on a label?
It means the solution contains 18 moles of H₂SO₄ in every 1 dm³ of solution.
This indicates a highly concentrated solution. -
How can we convert cm³ to dm³?
Divide the value in cm³ by 1000 to get dm³.
For example, 500 cm³ = 0.5 dm³. -
What is the molar mass of urea (CH₄N₂O)?
The molar mass of urea is 60 g/mol.
It is calculated from atomic masses: 14×2 + 12 + 16 + 1×4 = 60. -
How many moles are there in 40g of urea?
Moles = 40 ÷ 60 = 0.667 mol.
This is used to calculate the molarity of the solution. -
What is the molarity of a solution made by dissolving 40g of urea in 500 cm³ of water?
Molarity = 0.667 mol / 0.5 dm³ = 1.334 M.
It shows the concentration of the solution. -
How do you calculate molarity if moles are given?
Divide the number of moles by the volume in dm³.
For example, 0.05 mol in 600 cm³ → 0.05 ÷ 0.6 = 0.083 M. -
What is the molarity of a solution containing 0.05 mol KMnO₄ in 600 cm³ of water?
The molarity is 0.083 M.
Volume in dm³ is 0.6, so 0.05 ÷ 0.6 = 0.083. -
What is the molarity of 1.5 mol of KClO₃ in 250 cm³ of solution?
Convert 250 cm³ to 0.25 dm³, then use M = 1.5 ÷ 0.25 = 6 M. -
What is the molarity of 75g KClO₃ dissolved in 1.25 dm³ of solution?
First convert grams to moles using molar mass (K=39, Cl=35.5, O₃=48 → 122.5 g/mol).
Then M = moles ÷ 1.25 dm³. -
How do you find molarity if a 50 cm³ solution yields 0.25g of residue after evaporation?
Convert 0.25g to moles, then divide by 0.05 dm³ (50 cm³).
Molarity = moles ÷ 0.05. -
What is molarity and why is it important in chemistry?
Molarity is a way to express the concentration of a solution.
It tells us how many moles of solute are present in 1 dm³ of solution.
Chemists use molarity to determine how much of a substance is needed or present in reactions.
It allows accurate preparation of solutions and helps in calculating yields and stoichiometric ratios. -
How do you calculate molarity when given mass of solute and volume of solution?
First, convert the mass of the solute to moles using its molar mass.
Then convert the volume of solution from cm³ to dm³ by dividing by 1000.
Finally, use the formula:
Molarity (M) = moles of solute / volume of solution in dm³.
This gives the strength of the solution. -
Why do we convert cm³ to dm³ when calculating molarity?
Molarity is defined per dm³ (liter), not per cm³.
Since 1 dm³ = 1000 cm³, we divide the cm³ volume by 1000.
Using incorrect units can lead to wrong molarity values.
So, for accurate calculations, unit conversion is essential. -
What does a label showing “18M H₂SO₄” indicate about the solution?
It means the solution contains 18 moles of sulfuric acid per 1 dm³.
Such a solution is highly concentrated and reactive.
Labels like this help chemists understand how strong a solution is.
Handling concentrated acids like 18M H₂SO₄ requires extreme caution. -
How does molarity help in comparing concentrated and dilute solutions?
Molarity gives a numerical value to the concentration of a solution.
A higher molarity means more solute per volume — a concentrated solution.
A lower molarity means less solute per volume — a dilute solution.
This helps in comparing strengths of acids, bases, or any solution precisely. -
What is dilution of a solution?
Dilution is the process of making a solution less concentrated by adding more solvent, usually water. -
Which formula is used for dilution calculations?
The formula used is: M₁V₁ = M₂V₂, where M and V represent molarity and volume. -
What does M₁ represent in the dilution formula?
M₁ is the concentration of the original (concentrated) solution. -
What does V₂ represent in the dilution formula?
V₂ is the total volume of the final (diluted) solution. -
What is the purpose of using the dilution formula?
It helps calculate how much concentrated solution is needed to achieve a desired dilution. -
What is the molarity of concentrated HCl used in Activity 3.1?
The molarity of concentrated HCl used is 12 M. -
How much volume of 12 M HCl is required to prepare 250 cm³ of 0.1 M HCl solution?
Only 2.08 cm³ of 12 M HCl is required for the dilution. -
What apparatus is used in Activity 3.1 for preparing the solution?
A volumetric flask, graduated pipette, and distilled water are used. -
What is done after adding HCl to the volumetric flask in Activity 3.1?
Distilled water is added up to the mark and the solution is shaken well. -
What is the final concentration prepared in Activity 3.1?
The final concentration of the solution is 0.1 M HCl. -
What is the formula applied in Example 3.15 for dilution?
The same dilution formula M₁V₁ = M₂V₂ is used. -
What are the initial and final concentrations in Example 3.15?
Initial concentration is 4 M and final concentration is 1 M. -
What volume of 4 M KNO₃ is needed to make 500 cm³ of 1 M KNO₃?
You need 125 cm³ of 4 M solution to make 500 cm³ of 1 M solution. -
What is done after measuring 125 cm³ of KNO₃ solution?
It is mixed with water to make a total volume of 500 cm³. -
Why does adding water to a solution reduce its concentration?
Adding water increases the total volume, which spreads out the solute particles, making the solution less concentrated. -
How does the dilution formula help in laboratory work?
It allows accurate calculation of how much concentrated solution is needed to prepare a solution of desired strength and volume. -
Why is it important to use a volumetric flask in dilution?
It ensures precise measurement of the final volume, which is crucial for accurate dilution. -
Why must the solution be mixed well after dilution?
To ensure the solute is evenly distributed, giving a uniform concentration throughout the solution. -
What would happen if too much concentrated acid is added during dilution?
The resulting solution would be stronger than intended, affecting experimental accuracy and safety. -
Why is a graduated pipette used in dilution activities?
To measure small and accurate volumes of concentrated solutions, which are usually highly reactive. -
Why do we need to dilute concentrated acids like HCl before use?
Concentrated acids are too strong and dangerous; dilution makes them safer and usable for experiments. -
What does the product M × V represent in the dilution formula?
It represents the number of moles of solute, which remains constant during dilution. -
Can the same dilution formula be used for both acids and salts? Why?
Yes, because the formula is based on molarity and volume, not the type of solute. -
Why is molarity used instead of mass in dilution calculations?
Molarity relates directly to how many particles are present in a solution, which affects chemical reactions. -
What is the chemical equation used in this titration?
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l), showing acid-base neutralization. -
What is the volume of NaOH solution taken during titration?
10 cm³ of NaOH solution is taken using a pipette. -
How do you know when neutralization is complete?
When the pink color disappears, it indicates complete neutralization. -
Why are three concordant readings taken?
To ensure accuracy and consistency in the titration results. -
How is the volume of HCl used determined?
By subtracting the initial burette reading from the final burette reading. -
What is the molarity of NaOH solution used?
The molarity of NaOH is 0.1 M as given in the activity. -
How is the molarity of HCl solution calculated?
Using the formula: M₁V₁ / n₁ = M₂V₂ / n₂, with values from the titration. -
What is the molarity result of the HCl solution?
The molarity of the HCl solution is calculated as 0.1 M. -
How is the strength of HCl solution calculated?
Strength = Molarity × Molar Mass = 0.1 × 36.5 = 3.65 g/dm³. -
What is the unit of strength of an acid solution?
Strength is expressed in grams per dm³ (g/dm³). -
What is the result of titration in Concept Assessment 3.11 (i)?
Molarity of NaOH = (0.1 × 9.8) / 10 = 0.098 M. -
What is the amount of NaOH present per cm³ of solution?
Amount = Molarity × Volume = 0.098 × 1 = 0.098 mmol/cm³. -
How do you dilute 200 cm³ of 6 M HCl to 2 M?
Using M₁V₁ = M₂V₂: (6×200)/2 = 600 cm³ → add 400 cm³ water. -
How much of the original HCl solution is used in the dilution?
Only 200 cm³ of the original 6 M HCl is used. -
How much water should be added for the desired dilution?
400 cm³ of water must be added to make a total of 600 cm³. -
Why is phenolphthalein used as an indicator in titration?
It changes color at the endpoint of acid-base neutralization, helping detect when the reaction is complete. -
Why is NaOH solution placed in a conical flask during titration?
Because it is easier to swirl the flask and mix solutions properly while adding acid drop by drop. -
What is the importance of taking concordant readings in titration?
Concordant readings ensure the results are accurate and repeatable, minimizing experimental error. -
Why do we use a burette for the acid in titration?
The burette allows precise control over how much acid is added, drop by drop. -
What does the disappearance of the pink color indicate?
It indicates that the acid has completely neutralized the base present in the flask. -
Why is the mole ratio of HCl and NaOH taken as 1:1?
Because one mole of HCl reacts with one mole of NaOH in the balanced chemical equation. -
Why is molarity important in titration calculations?
Molarity shows how many moles of solute are present per unit volume, which helps determine chemical amounts in reactions. -
What does a titration experiment help us find?
It helps determine the unknown concentration of a solution by reacting it with a solution of known concentration. -
Why do we multiply molarity with molar mass to find strength?
Because strength in g/dm³ tells us how many grams of solute are present per liter of solution. -
Why do we dilute a concentrated solution like HCl?
To make it safer to handle and suitable for use in specific experiments or reactions.