Topic 3: Quantitative Chemistry

Question 1

What is the law of conservation of mass?

Answer 1

The law of conservation of mass states that no atoms are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants.

Question 2

Write a balanced equation of magnesium reacting with hydrochloric acid.

Answer 2

Mg(s) + 2 HCl(aq) → MgCl2(aq) + H2(g)

Question 3

Define relative atomic mass

Answer 3

RAM - average mass of atoms in an element taking into account masses and abundance of its isotopes, relative to 12C.

Question 4

Define relative formula mass.

Answer 4

RFM - sum of RAM’s of all atoms in the formula.

Question 5

What is the relative formula mass of:
A) CaF2
B) C6H12O6

Answer 5

CaF2 - (Ar values: Ca = 40, F = 19)
40 + 19 + 19 = 78
—
C2H12O6 - (Ar values: C = 12, H = 1, O = 16)
(12 x 6) + (1 x 12) + (16 x 6) = 180

Question 6

The following reaction occurs in a test tube under a Bunsen Burner:
4 MgO(s) + CH4(g) → 4 Mg(s) + 2 H2O(g) + CO2(g)
The carbon dioxide and water escape from the test tube. Use the equation to explain why.

Answer 6

They are both gases

Question 7

What is Avogadro’s constant?

Answer 7

The number of atoms, molecules or ions in a mole of a given substance. The value of the constant is 6.02 x 10^23.

Question 8

What is the formula that links mass, molecular mass and moles together

Answer 8

Mass = Mr x Moles

Question 9

What is the mass of: 20 moles of calcium carbonate, CaCO3

Answer 9

Mass = Mr x Moles
Mr = 100
100 x 20 = 2000 g

Question 10

Calculate the amount of carbon dioxide in moles in 0.32 g of carbon dioxide.

Relative atomic masses (Ar): carbon = 12, oxygen = 16

Answer 10

Moles = Mass / Mr
0.32 / 44 = 0.007

Question 11

Nitrogen and hydrogen form ammonia shown by the following equation:
N2(g) + 3 H2(g) ⇌ 2 NH3(g)
Calculate the mass of nitrogen needed to form 6.8 tonnes of ammonia.
Relative atomic masses (Ar): H = 1; N = 14

Answer 11

• Step 1 - Work out the number of number of moles of ammonia (Mr of ammonia = 17)

6800000 / 17 = 400000 moles of ammonia

• Step 2 - Use the balanced equation and number of moles of ammonia to work out the number of moles of nitrogen

The ratio of nitrogen to ammonia is 1:2

Therefore the number of moles of nitrogen is 400000/2 = 200000

• Step 3 - Work out the mass of nitrogen (Mr of N2 is 28)

200000 x 28 = 5600000 g = 5.6 tonnes.

Question 12

State what we mean by a limiting reactant in a chemical reaction

Answer 12

In a chemical reaction involving two reactants, it is common to use an excess of one of the reactants to ensure that all of the other reactant is used. The reactant that is completely used up is called the limiting reactant because it limits the amount of products.

Question 13

Hydrogen peroxide decomposes in water to form water and oxygen. How many grams of oxygen gas will be given off from 40.8 g of hydrogen peroxide?

Answer 13

• Step 1: Write the balanced equation 2 H2O2(l) → 2 H2O + O2(g)
  Mr of H2O2 = 34
• Step 2: Number of moles in 40.8 g : 40.8/34 = 1.2 moles

Ratio in the balanced equation of H2O2 : O2 = 2:1

• Step 3 :Therefore number of moles of O2 = 0.6 moles
• Step 4: Mass of oxygen = 0.6 x 32 (Mr of O2) = 19.2

Question 14

Write down the two formulae that link concentration, mass and volume together.

Answer 14

• Concentration (g per dm3) = Mass (g)/Volume (dm^3)
• Concentration (mol per dm^3) = nr of moles/volume (dm^3)

Question 15

31.0 cm3 of potassium hydroxide solution neutralised 25.0 cm3 of 2.0 moldm−3 nitric acid.

HNO3 + KOH → KNO3 + H2O

Calculate the concentration of the potassium hydroxide solution in moldm^-3

Answer 15

• Step 1: Calculate the moles of HNO3 used = Concentration x volume

2 x 0.025 dm3 (25/1000 to convert the units) = 0.05 moles

• Step 2 : Calculate the moles of KOH
  Ratio is 1:1 therefore number of moles of KOH = 0.05
• Step 3 : Calculate the concentration of KOH Volume = Moles/concentration; 0.05 / 0.031 = 1.61

Question 16

What is the molar volume of a gas at room temperature and pressure?

Answer 16

1 mole of a gas at room temperature and pressure occupies 24 dm^3

Question 17

What is titration?

Answer 17

A technique for finding the concentration of a solution by reacting a known volume of this solution with a solution of known concentration.

Question 18

How do you conduct a titration?

Answer 18

a) Rinse the pipette with a solution of unknown concentration. Use the pipette to measure out the known volume of this solution.

b) Add an indicator (a substance that changes colour at the end of titration)

c) Rinse the burette with a solution of known concentration. Discard the liquid. Use a burette to gradually add the solution of a known concentration.

d) When indicator changes colour (at the end point), the volume added is
recorded

e) It is important to get concordant volume results - they have to lie close to eachother

f) Suitable calculations are performed to find the concentration.

Question 19

Why is it not always possible to obtain the theoretical amount of product in a chemical reaction?

Answer 19

● The reaction may not go to completion because it is reversible.

● Some of the product may be lost when it is separated from the reaction mixture.

● Some of the reactants may react in ways different to the expected reaction (side reactions may occur).

Question 20

What is the percentage yield of a product in a chemical reaction? State equation.

Answer 20

% Yield = Actual mass of a product
—————————————— x 100
Maximum theoretical mass
of product

Question 21

What is the % yield of NH3 if 40.5 g NH3 is produced from 20.0 mol H2 and excess N2?

Answer 21

• Step 1 - Write the balanced equation

N2 + 3 H2 → 2 NH3

• Step 2 - Calculate the theoretical amount of NH3. Moles NH3 (ratio of H2 to NH3 is 3:2); of 20/1.5 = 13.3 moles

13.3 X 17 (Mr of NH3) = 227

• Step 3 - Calculate percentage yield of NH3

40.5/227 x 100 = 17.8%

Question 22

What is atom economy?

Answer 22

• A measure of the amount of starting materials that end up as useful products.
• It is a ratio of the relative formula mass of desired product to the sum of relative formula masses of reactants.

Question 23

Look at the equations for the two reactions that produce CuCl2:

Reaction I: CuCO3(s) + 2 HCl(aq) → CuCl2(aq) + H2O(l) + CO2(g)

Reaction II: CuO(s) + 2 HCl(aq) → CuCl2(aq) + H2O(l)

Reactive formula masses: CuO = 79.5; HCl = 36.5; CuCl2 = 134.5; H2O = 18

Which reaction has a better atom economy?

Answer 23

Reaction II (look at the reactants):

Total formula mass of reactants=152.5

Formula mass of CuCl2 = 134.5

(134.5/152.5) x 100% = 88.2%