C3 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) + 2HCl (aq) → MgC2 (aq) + H2 (g)

Question 3

Define relative atomic mass and relative formula mass.

Answer 3

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

RFM - sum of RAMs’ of al atoms ni the formula.

Question 4

What is the relative formula mass of:

A) CaF2
B) C6H12O6

Answer 4

CaF2 -(Ar values: Ca =40, F= 19) 40 + 19 + 19 = 78

CH12O6 -(Ar values: C=12, H=1,0=16)
(12 x 6) + (1 x 12) + (16 x 6) = 180

Question 5

The following reaction occurs in a test tube under a Bunsen Burner:

4MgO (s) + CH4 (g) → 4Mg (s) + 2H20 (g) + CO2 (g)

The carbon dioxide and water escape from the test tube.

Use the equation to explain why.

Answer 5

They are both gases

Question 6

Suggest how you could increase the precision

Answer 6

Measure to more decimal places or use a more sensitive balance / apparatus

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 si 6.02 x10^23.

Question 8

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

Answer 8

Mass = Mr x Moles

Question 9

Write the equation that links Mass, Mr and Moles

Answer 9

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

Question 10

What si the mas of 20 moles of calcium carbonate, CaCO3

Answer 10

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

Question 11

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

Relative atomic masses (Ar): carbon = 12, oxygen = 61

Answer 11

Moles = Mass / Mr
0.32 / 4 = 0.007

Question 12

Nitrogen and hydrogen form ammonia shown by the following equation:

N2 (g) +3H2 (g) ⇌ 2 NH3 (g)

Calculate the mass of nitrogen needed ot form 6.8 tonnes of ammonia.

Relative atomic masses (Ar): H = 1; N = 14

Answer 12

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 ot 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 mas of nitrogen M(r of N, si 28)

200000 x 28 = 5600000 g = 5.6 tonnes.

Question 13

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

Answer 13

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 ti limits the amount of products.

Question 14

How much oxygen will be given of from 40.8 g of hydrogen peroxide?

Answer 14

Step 1: Write the balanced equation 2 H20 2(l)) →2 H202 = 34

Step 2: Number of moles in 40.8 g: 40.8/34 = 1.2 moles
Ratio in the balanced equation of H202 : 02 = 2:1

Step 3: Therefore number of moles of 0, = 0.6 moles

Step 4: Mass of oxygen = 0.6 × 32 ( Mr of 02) = 19.2

Question 15

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

Answer 15

Concentration (g per dm3) = Mass (g)/Volume (dm3)

Concentration (mol per dm3) = no of moles/volume (dm3)

Question 16

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

HNO3 + KOH → KNO + H20

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

Answer 16

Step 1: Calculate the moles of HNO3 used = Concentration x volume
2 x 0.025 dm (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 17

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

Answer 17

1 mole of agas at rom temperature and pressure occupies 24 dm3

Question 18

What is titration?

Answer 18

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

Question 19

How do you conduct a titration?

Answer 19

1) Rinse the pipette with a solution of unknown concentration. Use the pipette to measure out the known volume of this solution.
2) Ad an indicator (a substance that changes colour at the end of titration)
3) Rinse the burette with a solution of known concentration. Discard the liquid. Use a burette to gradually add the solution of a known concentration.
4) When indicator changes colour (at the end point), the volume added is recorded
5) It is important to get concordant volume results - they have to lie close to each other
6) Suitable calculations are performed to find the concentration.

Question 20

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

Answer 20

• 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 21

How is the percentage yield of a product in a chemical reaction?

Answer 21

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

Question 22

What si the % yield of NH, fi 40.5 g NH3 is produced from 20.0 mol H2 and excess N2?

Answer 22

Step 1 - Write the balanced equation
N2 +3H2 →2NHg

Step 2 - Calculate the theoretical amount of NH, Moles NH (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 × 100 = 17.8%

Question 23

What is atom economy?

Answer 23

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 24

Look at the equations for the two reactions that produce CuCl2

Reaction I: CuCO3 (s) + 2 HCL (aq) + H20 (l) + CO2 (g)

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

Reactive formula masses: CuO = 79.5; HCI = 36.5; CuCI, = 134.5;
H20 = 18

Which reaction has a better atom economy?

Answer 24

Reaction II (look at the reactants):
Total formula mass of reactants = 152.5
Formula mas of CuCl2 = 134.5 (134.5/152.5) x100% = 88.2%

Question 25

Avogadro constant:

Answer 25

The number of atoms, molecules or ions in a mole of a given substance.

Question 26

Avogadro’s law:

Answer 26

Equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure.

Question 27

Concentration:

Answer 27

​The amount of substance (e.g. the mass) in a certain volume of a solution.

Question 28

Conservation of mass:

Answer 28

​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 29

Limiting reactant:

Answer 29

The reactant that is completely used up since it limits the amount of products formed.

Question 30

Mole:

Answer 30

Chemical amounts are measured in moles. The mole is the unit for amount of substance. The symbol for the unit mole is mol.

Question 31

Relative formula mass:

Answer 31

The sum of the relative atomic masses of the atoms in the numbers shown in the formula. It is numerically equal to the mass of one mole of a substance in grams.

Question 32

Thermal decomposition:

Answer 32

The reaction that​ occurs when heat is applied to a compound causing it to break down into its different chemical constituents.

Question 33

Uncertainty:

Answer 33

All measurements have a degree of uncertainty regardless of precision and accuracy. Uncertainty can be due to the limitations of the measuring equipment or due to the skill of the experimenter carrying out the measurements