Amount of Substance

Question 1

Ionic equations

Answer 1

First disassociate all of the ions and cancel the ions that appear on both sides

Question 2

Relative atomic mass

Answer 2

The average mass of one atom compared to one twelfth of the mass of one atom of carbon-12

Question 3

Relative molecular mass

Answer 3

Relative molecular mass is the average mass of a molecule compared to one twelfth of the mass of carbon-12.

Question 4

What equation links mass, moles and Mr?

Answer 4

Moles = mass/Mr

Question 5

What equation links moles, concentration and volume?

Answer 5

Moles = concentration x volume (dm³)

Question 6

What is the ideal gas equation?

Answer 6

P(Pressure in pascals)V(volume in m³ ) = n(The number of moles)R(The gas constant 8.314)T(Temperature in kelvin)

Question 7

How do you work out how many atoms you have of an element when you have the Mr and the mass?

Answer 7

Moles = mass/Mr

Moles x Avogadro constant (6.02 x 10²³)

Question 8

Equation involving mass, density and volume?

Answer 8

Density = mass/volume

Question 9

Empirical formula

Answer 9

An empirical formula is the simplest ratio of atoms of each element in a compound.

Question 10

Molecular formula

Answer 10

A molecular formula is the actual number of atoms of each element in the compound.

Question 11

What is percentage yield and how do you calculate it?

Answer 11

percentage yield = (actual yield/theoretical yield) x 100

Question 12

What is percentage atom economy?

Answer 12

(Mr of useful products/total Mr of reactants) x 100

Question 13

Why are all Mr values related to Carbon-12?

Answer 13

It is the agreed standard.

Question 14

Give 3 reasons why the yield of an experiment might be lower than the expected value?

Answer 14

Other reactions are taking place

The reaction is reversible

The reactants were impure

Reactants/products still on apparatus

Question 15

In a titration why is the volumetric flask containing your standard solution inverted multiple times?

Answer 15

To ensure that the concentration is equal throughout.

Question 16

If you have already rinsed the burette and pipette with water why do you need to rinse them with solution?

Answer 16

The water could stick to the glassware and dilute the solution that is being added so by rinsing with the solution this will not change the concentration.

Question 17

Why is it important to bleed the air out of the burette when doing a titration?

Answer 17

If any air remains in the burette it shows a false initial reading as no solution actually leaves the burette until later.

Question 18

Why should you not force the solution out of the pipette with the filler if it is faster?

Answer 18

The filler leaves a small volume of solution at the bottom of the pipette which is accounted for which will occur due to capillary action from gravity. It could also damage the pipette.

Question 19

Why should you not use lots of indicator in a titration?

Answer 19

Indicators in themselves are acidic (weak) and in order to get a colour change you really don’t need that much, so adding more than is needed just increase the volume of titrant needed to add to the solution.

Question 20

Why is the conical flask held over a white tile?

Answer 20

The contrast between the white tile and the solution makes it easier to see an endpoint on the colour change.

Question 21

How close do results need to be together to be concordant?

Answer 21

0.1cm³ of each other and you only need 2 to take a mean average

Question 22

Given that the uncertainty of a titration came mainly from the burette how could you reduce the percentage uncertainty?

Answer 22

Use a more diluted acid

Question 23

What are the potential errors when using a gas syringe?

Answer 23

Gas escapes before bung is inserted
Syringe sticks

Question 24

How to decrease the percentage uncertainty when measuring masses?

Answer 24

Weigh before and after to find mass transferred

Question 25

Why would a manufacturer want to produce something with less steps and a higher percentage atom economy?

Answer 25

Increases yield

decreases waste