1.3 Chemical Calculations

Question 1

What does the term relative atomic mass mean?

Answer 1

The average mass of one atom of an element compared to 1/12th the mass of an atom of carbon-12.

Question 2

What does the term relative formula mass mean?

Answer 2

The weighted average of the masses of the formula units compared to 1/12th the mass of an atom of carbon-12.

Question 3

What does the term relative molecular mass mean?

Answer 3

The weighted average of the masses of the molecules compared to 1/12th the mass of an atom of carbon-12.

Question 4

What does the term relative isotopic mass mean?

Answer 4

The mass of one atom of an isotope compared to 1/12th the mass of an atom of carbon-12.

Question 5

What is mass spectrometry?

Answer 5

Mass spectrometry is a type of analytical technique which measures the mass to charge ratio of ions. It also measures how abundant each ion is so it can be used to calculate the relative atomic mass.

Question 6

Explain the five stages of TOF mass spectrometry

Answer 6

1. Ionisation - vapourised sample is injected into the mass spectrometer where it is ionised.
2. Acceleration - the positively charged ions are accelerated towards a negatively charged detection plate.
3. Ion drift - the ions are deflected into a curved path with a magnetic field.
4. Detection - the positive ions are detected by hitting a negatively charged
   plate, where they gain an electron, producing a current. The greater the
   current, the greater the abundance.
5. Analysis - the relative abundance of each isotope are compared. Relevant
   calculations are made.

Question 7

Answer 7

Question 8

How can you use a mass spectrum to deduce the relative molecular mass of a sample of a compound?

Answer 8

The peak with the highest m/z value (the molecular ion peak, M+) is caused by the whole molecule, therefore m/z value = molecular mass.

Question 9

Chlorine has two possible isotopes, Cl35 with a 75% abundance and Cl37 with a 25% abundance. Predict what the mass spectrum would look like for the diatomic molecule

Answer 9

Question 10

Define empirical formula

Answer 10

The simplest whole number ratio of atoms of each element in a compound.

Question 11

Define molecular formula

Answer 11

The actual number of atoms of each element present in a compound.

Question 12

What is the empirical formula of Fe2O4?

Answer 12

FeO2

Question 13

A molecule has the empirical formula C4H3O2 and a relative molecular mass of 166. What is the molecular formula?

Answer 13

Empirical mass = 4(12) + 3(1) + 2(16) = 83
Relative molecular mass = 166
Compare molecular mass with empirical mass = 166 / 83 = 2
So the molecular formula is double the empirical formula: C8H6O4

Question 14

Define the Avogadro constant

Answer 14

The number of particles in one mole of a substance. This is 6.02 x 10^23 particles.

Question 15

What is the equation linking Avogadro’s constant to moles?

Answer 15

Number of particles =
Moles x Avogadro’s constant

Question 16

There are 4.816 x 1024 atoms of iron in a sample. How many moles of iron are in the sample?

Answer 16

Avogadro’s constant = 6.02 x 1023 mol-1
Moles = number of atoms ÷ Avogadro’s constant
= (4.816 x 1024) ÷ (6.02 x 1023)
= 8 mol of Fe

Question 17

Define molar mass

Answer 17

The mass per mole of a substance, measured in g mol-1.

Question 18

What is the equation that links moles to mass?

Answer 18

Moles =
Mass (g) / Relative atomic mass

Question 19

Define concentration

Answer 19

The amount of moles per unit volume.

Question 20

How can concentration be calculated in g/dm3?

Answer 20

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

Question 21

How can concentration be calculated in mol/dm3?

Answer 21

Concentration (mol/dm3) = Moles / Volume (dm3)

Question 22

5.00 g of NaCl is dissolved in 25 cm3 of water. Calculate the concentration of the solution in mol/dm3.

Answer 22

Moles of NaCl = 5 / 58.5 = 0.0855
Volume in dm3 = 25 / 1000 = 0.025
Concentration in mol/dm3 = 0.0855 / 0.025 = 3.42 mol/dm3

Question 23

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

Answer 23

24 dm3

Question 24

What is RTP

Answer 24

Room temperature and pressure:
- 293 K
- 101 kPa

Question 25

What equation links molar volume at RTP to moles?

Answer 25

Volume of gas at RTP (dm3) = moles x 24

Question 26

How many moles of oxygen are in 72 dm3 at RTP?

Answer 26

Moles = volume / 24
= 72 / 24
= 3 moles

Question 27

How is the molar volume at a given pressure affected if temperature is increased?

Answer 27

An increase in temperature reduces the molar volume at a given temperature.

Question 28

How is the molar volume at a given temperature affected if pressure is increased?

Answer 28

An increase in pressure will mean the maximum molar volume reduces. Up to this maximum, increasing the pressure will increase the molar volume.

Question 29

What is the ideal gas law?

Answer 29

pV = nRT
p - pressure (Pa)
V - volume (m3)
n - number of moles
R - gas constant (8.31 J K-1mol-1) T - temperature (K)

Question 30

At a temperature of 55.0 oC and a pressure of 275 kPa, a gas occupies a volume of 1.10 dm3. How many moles of the gas are present? The gas constant is 8.31 J K-1mol-1.

Answer 30

pV = nRT
- 275 kPa is 275000 Pa
- 55.0 oC is 328 K
- 1.10 dm3 is 0.001 m3

Therefore,
n=(pV)/(RT)
n=(275000 x 0.001)/(8.31 x 328) n= 0.10 moles

Question 31

How can percentage yield be calculated?

Answer 31

Percentage yield =
(Actual yield ÷ Theoretical yield) x 100

Question 32

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

Answer 32

Equation: N2 + 3H2 → 2NH3
Moles of ammonia = 20/1.5 = 13.3 moles
Mass of ammonia = 13.3 x (14+1+1+1) = 227 g Percentage yield = (40.5/227) x 100 = 17.9%

Question 33

Why might the actual yield of product be less than expected?

Answer 33

• Incomplete reaction
• Unwanted side reactions
• Practical losses, for example some solid
  may get lost when being transferred
  between beakers

Question 34

What is the atom economy of a reaction?

Answer 34

Atom economy is a measure of the efficiency of the reaction. It looks at the amount of reactants that get turned into useful products.

Question 35

How can atom economy be calculated?

Answer 35

Atom economy=
(Mr of desired product ÷ total Mr of reactants) x 100

Question 36

Fill in the gap: ‘The _____ the atom economy, the more sustainable and efficient the process’

Answer 36

Higher

Question 37

How can percentage error be calculated?

Answer 37

% error = (absolute uncertainty / calculated value) x 100

Question 38

A titre volume is recorded as 11.30 cm3. The accuracy of the burette is ±0.05 cm3. Calculate the maximum percentage error.

Answer 38

% error = absolute uncertainty x 100 calculated value
(0.05 ÷ 11.30) x 100 = 0.442%