Moles - TN

Question 1

Calculate the relative formula mass of Fe2O3
Fe = 56
O = 16

Answer 1

56 x 2
16 x 3
=160

Question 2

what is a mole

Answer 2

The unit for an amount of substance. 1 mol is the amount of substance that contains the same number of particles as there are atoms in 12.0 g of carbon-12

Question 3

What is Avogadro’s constant

Answer 3

The number of atoms in one mole of a substance. Its value is 6.02 × 10^23

Question 4

Equation for: how many moles are in this substance?

Answer 4

moles = mass/Mr

Question 5

What is Relative Atomic Mass

Answer 5

The weighted average mass of all the isotopes of an element

Question 6

What is Relative Molecular Mass

Answer 6

The total average mass of one molecule of a compound

Question 7

What is the Empirical Formula

Answer 7

The simplest whole number ratio of atoms in a compound

Question 8

Calculate the EMPIRICAL FORMULA of a compound which contains 92.3% carbon and 7.7% hydrogen
(would be same calculation if given 7.8g carbon and 3.1g hydrogen)

Answer 8

1) Take the percentage (or mass if you are given it) and divide each by the Ar of the element TO FIND MOLES
92.3/12 = 7.69 7.7/1 = 7.7
2) Determine the ratio of the atoms’ moles by dividing both by the smallest number
7.69/7.69 = 1 7.7/7.69 = 1.0013

CH

Question 9

What mass if formed when 54.75g of HCl is reacted with sufficient Mg?
REACTING MASSES

Answer 9

Mg + 2HCl ==== MgCl2 + H2
54.75g ?g
1)Find no moles in compound of known mass
54.75/ 1+35.5 ==1.5
Find the ratios
HCl : MgCl2
2 : 1 - from equation
1.5 : 0.75 - moles worked out

3) find mass of MgCl2
0.75 x (24 +(2 x 35.5))
= 71.25g

Question 10

Percentage Yield equation

Answer 10

Percentage yield = actual yield/theoretical yield x100

Question 11

Why will percentage yield never be 100%

Answer 11

• mass could be lost to surroundings
• the substance might not be pure
• incomplete reaction (eg. insufficient heat provided)

Question 12

Describe the experiment to determine how many water of crystallisation CuSO4 has

Answer 12

Weigh crucible
Weigh crucible + CuSO4-xH20
Heat crucible
Weigh crucible + CuSO4

Determine mass of CuSO4 and H2O lost
Find moles of each
Find ratio of H20 to CuSO4
CuSO4 : H2O
1 : x

x = number of waters of crystallisation
eg. CuSO4-5H2O

Question 13

what is a molecular formula

Answer 13

the actual number of atoms of each element in a compound

Question 14

How to find which substance is limiting or in excess

10g of Mg mixed with 10g HCl
Mg + 2HCl -> MgCl2 + H2

Answer 14

1) Find the moles of both Mg and HCl by using moles = mass/Mr and the ratios
2) Find mass of Hcl NEEDED by doing moles x Mr
3) This means that to react with 10g Mg we would need 30.4g HCl

BUT we only have 10g HCl so it is NOT ENOUGH so it is LIMITING and
Mg is in EXCESS

Question 15

LIMITING REAGENTS
S + 3T -> 2W
with 3mol S and 6mol T, how many W can you make?

Answer 15

S should need 9mol T because
S : T
1 : 3
3 : 9

so with only 6mol T, only 2mol S can react

ratio 1: 3 : 2
moles 2:6:4

4MOL W

Question 16

what equation would you use to find the percentage of a substance in its compound (eg % of C in C3H8)

Answer 16

%C = {mass of C / RFM(C3H8)} x 100

Question 17

water of crystallisation - information

Answer 17

Hydrated salt crystals contain water trapped inside them in a fixed mole ratio.
This water is called water of crystallisation, and it’s written into a formula like:
CuSO4.5H2O

When hydrated salt crystals are heated, they lose their water:
Hydrated copper sulphate (CuSO4.5H2O) → Anhydrous copper sulphate Water (vapour)