PAG 01 – Moles Determination: Module 1: Practical Skills in Chemistry: Chemistry OCR A A Level

How can the number of moles of CO2 be calculated from the volume of CO2 collected during an experiment?

- If at room temperature and pressure (RTP), use the equation n = V/24 where V is the volume in dm^3 .

- If not at RTP, rearrange the ideal gas law PV = nRT and plug in the numbers for R (8.31), T in Kelvin, P in Pascals and V in m^3.

What is used to accurately measure mass?

A digital balance that records mass to 2 decimal places.

Give 3 possible sources of error when investigating the volume of gas produced in a reaction.

1. Some gas could have escaped before the bung is put in the conical flask.

2. The reaction may be incomplete.

3. Some gas may dissolve in the water meaning a smaller volume would collected than was released.

How can measuring the volume of gas produced during the reaction between magnesium and sulfuric acid be used to determine the relative atomic mass of magnesium?

- Measure the mass of some magnesium then react it with sulfuric acid.

- Measure the volume of hydrogen gas produced in a gas syringe or an upturned water-filled measuring cylinder.

- Calculate the number of moles of H_2 produced (number of moles = volume / 24 dm^3).

- Use the ratio in the chemical equation to find the number of moles of magnesium.

- Since the original mass of magnesium is known, the relative atomic mass can be calculated by dividing mass of magnesium by the number of moles.

Give 2 possible causes of sources in this experiment.

- Some hydrogen gas may escape before the bung is put in the conical flask.

- Some magnesium may not react.

Why is it important to rapidly put the bung in the conical flask after adding the sulfuric acid?

The reaction starts as soon as the acid is added so H_2 will start to be produced. To reduce the amount of H_2 that escapes, the bung should be inserted rapidly.

What equation links moles, molecular mass and mass?

Mass (grams) = molecular mass x number of moles (mol)

m = M x n

What safety precautions should be taken when conducting an experiment with magnesium and sulfuric acid?

- Sulfuric acid cause skin and eye irritation so wear safety goggles and avoid contact with skin.

- Magnesium is highly flammable so keep away from naked flames.