PAG 1: Moles Determination

Question 1

What pieces of equipment can be used to measure volumes of gases? (2)

Answer 1

A gas syringe;

An upside down measuring cylinder filled with water

Question 2

Which reactions can be monitored using measurement of gas evolved? (1)

Answer 2

Reactions which produce gas as a product

Question 3

What are the steps in determining volume of gas evolved using a gas syringe? (4)

Answer 3

1. Add the reaction mixture to a conical flask;
2. Immediately connect the gas syringe with a rubber bung in the conical flask;
3. The evolved gas pushes the syringe plunger along;
4. Record the volume of gas evolved in the reaction

Question 4

Give two possible errors in gas volume measurement using a syringe. (2)

Answer 4

The syringe could stick;

Gas could escape from the flask before the bung is attached

Question 5

Why might gas volume not be measured accurately with certain gases? (2)

Answer 5

Gases like CO₂ and SO₂ are soluble in water;

So the measured amount of gas will be lower than the true amount

Question 6

What can the volume of gas collected be used to determine? (1)

Answer 6

The moles of gas produced; or the rate of reaction

Question 7

What is the purpose of heating a hydrated salt in a crucible? (1)

Answer 7

To determine the water of crystallisation

Question 8

What is the first step in the crucible heating method? (1)

Answer 8

Weigh an empty crucible on a balance

Question 9

What are the steps to determine water of crystallisation? (5)

Answer 9

1. Weigh an empty crucible on a mass balance;
2. Add a hydrated salt into the crucible, and weigh the salt and crucible together;
3. Place the crucible on a pipe-clay triangle above a bunsen burner;
4. Heat the crucible with the bunsen burner;
5. Weigh the crucible with the anhydrous salt

Question 10

Why is the crucible placed in a pipe-clay triangle? (1)

Answer 10

To keep the crucible stable during heating

Question 11

What should be done after heating the hydrated salt? (1)

Answer 11

Leave the crucible to cool; then weigh it with the anhydrous salt

Question 12

How is the mass of hydrated salt calculated? (1)

Answer 12

Mass of crucible and hydrated salt minus mass of crucible

Question 13

How is the mass of anhydrous salt calculated? (1)

Answer 13

Mass of crucible and anhydrous salt minus mass of crucible

Question 14

How is the mass of water lost calculated? (1)

Answer 14

Mass of hydrated salt minus mass of anhydrous salt

Question 15

How are the moles of water and salt used to deduce the formula of the salt? (1)

Answer 15

By dividing the moles of each and simplifying the ratio