1.3 Chemical Calculations

WJEC A-Level Chemistry

What is the relative isotopic mass?

The mass of an atom of an isotope relative to one-twelfth the mass of one atom of carbon-12.

e.g. ³⁵Cl has a relative isotopic mass of 35

What is the relative atomic mass (Ar)?

The average mass of one atom of the element relative to one-twelfth the mass of one atom of carbon-12.

• found on a periodic table

What is the relative formula mass (Mr)?

The average mass of a molecule relative to one-twelfth the mass of one atom of carbon-12.

• add up all the Ar

e.g. CaCO₃ 40.1 + 12 + (16×3) = 100.1

What is a mole?

One mole is the amount of substance that contains the same number of particles as there are atoms of carbon in exactly 12g of carbon-12.

Mass =

Mr x moles

What is the molar mass?

One mole of any substance has a mass equivalent to the Mr of that substance in grams (gmol^-1).

e.g. MgCl₂ - Mr=95.3, molar mass = 95.3 gmol^-1

What is the empirical formula?

Simplest formula of a substance

1. symbol of elements

2. % or mass

3. Ar

4. moles

5. divide by smallest mole

6. ratio

How to calculate reacting masses?

1. balanced equation

2. moles of one

3. ratio for moles of what we need

4. mass=mrxmoles

Percentage yield =

mass of product obtained / maximum theoretical mass x 100

Why will percentage yield always be less than 100%?

Due to loss of product during transfer,separation or an incomplete reaction

Atom economy =

mass of required product / total mass of reactants x 100

Concentration (moldm^-3) =

moles / volume (dm³)

cm³ to dm³

/1000

What is neutralisation?

• When the number of moles of H⁺ ions from the acid is equal to the number of OH^- ions form the alkali

• If an acid and alkali have different concentrations then a different volume of each is needed

1. moles

2. ratio

3. concentration/volume

What is molar volume?

The volume of one mole of gas is always the same whatever the gas as long as the temperature and pressure are constant

How much volume does 1 mole of gas occupy at standard temperature and pressure (stp 0°C)

22.4dm³

How much volume does 1 mole of gas occupy at room temperature and pressure (rtp 20°C)

24.0dm³

molar volume =

volume / moles

What is the ideal gas equation?

What is the gas equation under changing conditions?

Percentage error =

maximum error / quantity measured x 100

What happens to percentage error on burretes or for temperature change?

Must be doubled

What is a mass spectrometer?

an instrument to accurately determine the relative atomic/isotopic/formula mass.

What does a mass spectrometer measure?

• mass of each different isotope of the element

• relative abundance of each isotope of the element

What are the stages of the mass spectrometer?

1. vaporisation

2. ionisation

3. acceleration

4. deflection

5. detection

What happens during vaporisation stage of mass spectrometer?

liquid sample is vaporised into a gas

What happens during ionisation stage of mass spectrometer?

beam of electrons knock electrons from atoms or molecules in sample, forming positive ions

What happens during acceleration stage of mass spectrometer?

ions accelerated so all ions have same kinetic energy

What happens during deflection stage of mass spectrometer?

deflected according to mass and charge

• heaver ions are deflected less than light ones

• 2⁺ ions are deflected twice as much as 1⁺ ions

What happens during detection stage of mass spectrometer?

• on striking the detector ions accept electrons, lose their charge and create a current

• current created is proportional to the abundance of each ion

What are the uses of mass spectrometry?

• identifying trace compounds in forensic science

• identifying unknown compounds e.g. banned substances in athletes

• analysing molecules in space

What are the conditions of mass spectrometry?

vacuum so air molecules don’t interfere with moving ions

Mass spectra of elements

Double charged ions

• greater charge on ion means more deflection

• double charge = half mass

• smaller abundance as double ionisation is rare

Where are the peaks on the mass spectra of chlorine molecule (Cl₂)

• at m/z 70 due to ³⁵Cl — ³⁵Cl

• at m/z 72 due to ³⁵Cl — ³⁷Cl and ³⁷Cl — ³⁵Cl

• at m/z 74 due to ³⁷Cl — ³⁷Cl

Probability of peaks on the mass spectra of chlorine molecule (Cl₂)

• 70 - ¾ x ¾ = 9/16

• 72 - ¾ x ¼ + ¼ x ¾ = 6/16

• 74 - ¼ x ¼ = 1/16

What is a standard solution used for?

titration to find concentration of another solution

How to make a standard solution?

1. transfer all solid into a beaker, add water until it all dissolves

2. pour into a volumetric flask through a funnel

3. fill rest of flask to graduation mark with water

4. add stopper to flask and shake vigorously

Standardisation of an acid solution

Na₂CO₃ + 2HCl —> 2NaCl + CO₂ + H₂O

• Na₂CO₃ = standard solution

Titration calculations

• concordant results = +— 0.20cm³

• burette reading = +— 0.05cm³

• don’t include rough titration when calculating mean titre