C3 - Amount of Substance

Question 1

What is a mole?

Answer 1

A mole/mol (n) is a measure of the amount of a substance.
It is the amount of substance that contains
6.02 x 10 ^23 particles.

It can refer to anything, not just atoms

Question 2

What is the Avogadro constant?

Answer 2

(N’a) is the number of particles in each mole of carbon - 12

It is: 6.02 x 10 ^23

Question 3

What is the equation linking moles, mass and molecular mass?

Answer 3

Moles = mass (g) / molecular mass

N = m / Mr

Question 4

What is the empirical formula?

Answer 4

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

Question 5

How is empirical formula calculated?

Answer 5

1) Start with the number of grams of each element, given in the problem.
(If percentages are given, assume that the total mass is 100 grams so that the mass of each element = the percent given. )

2) Convert the mass of each element to moles using the molar mass from the periodic table. (divide the mass by atomic mass)
3) Divide each mole value by the smallest number of moles calculated.

4) Round to the nearest whole number.
This is the mole ratio of the elements and is represented by subscripts in the empirical formula. (If the number is too far to round (x.1 ~ x.9), then multiply each solution by the same.)

Question 6

How is molecular formula calculated?

Answer 6

1) Convert masses of substances into moles of atoms using n = m/M
2) Find the smallest whole number ratio and empirical formula
3) Write the relative mass of the empirical formula
4) Find the number of units of the formula in one molecule by dividing the Mr of the compound by the formula mass
5) Write the molecular formula

Question 7

What is a hydrated salt?

Answer 7

One which has water as part of its crystalline structure, known as water of crystallisation.

Question 8

What is the equation linking moles, concentration and volume?

Answer 8

moles = volume * concentration

n = c * V
mol = moldm^-3 * dm^3

Volume is usually measured in cm3 so you need to convert to dm3 by dividing by 1000

n = c * V (cm3) / 1000

Question 9

What is the molar volume of gas?

Answer 9

One mole of any gas has a volume of 24dm^3 or 24,000cm3 at RTP.

Volume of gas = number of moles * 24

Question 10

What is RTP?

Answer 10

Room temperature & pressure
RTP is:
20 degrees C
101kPa

Question 11

What equation links moles, volume and molar gas volume?

Answer 11

moles = volume (dm^3) / molar gas volume (24)

moles = volume (cm^3) / molar gas volume (24,000)

Question 12

What is the ideal gas equation?

Answer 12

pV = nRT

p = pressure in Pa
V = volume in m^3
n = moles
R = constant 8.31
T = temperature in K

This relies on the assumptions that forces between molecules are negligible and gas molecules have a negligible size compared to the size of the container.

Question 13

What is stoichiometry?

Answer 13

In a balanced equation, the balancing numbers give the ratio of the amount in moles of each substance.

Question 14

How can quantities from amounts be calculated from equations?

Answer 14

Work out the amount in moles of whatever you can

Use the equation to work out the amount in moles of the unknown chemical

Work out the unknown information required

Question 15

What is percentage yield?

Answer 15

The conversion of starting materials into a desired product.
Maximum possible amount obtained is the theoretical yield.
This is hard to achieve and actual yield is usually lower than the theoretical.

Question 16

Why does actual yield differ from theoretical yield?

Answer 16

The reaction may not have gone to completion

Other reactions may have taken place alongside the main reaction

Purification of the product may result in loss of some problem

Question 17

How is percentage you calculated?

Answer 17

% yield = (actual yield / theoretical yield) * 100

Question 18

What is a limiting reagent?

Answer 18

The reactants that is NOT in excess will be completely used up first and stop the reaction, known as the limiting reagent.

The limiting reagent can be calculated by working out the amount in moles of each reactant and comparing with the equation

Question 19

What is atom economy?

Answer 19

The atom economy of a chemical reaction is a measure of how well atoms have been utilised

High atom economies produce a large proportion of desired products, few waste products and are important for sustainability as they make the best use of natural resources.

Question 20

How is atom economy calculated?

Answer 20

Atom economy = (sum of molar masses of desired products / sum of molar masses of ALL products) * 100