Topic 5a - Calculations

Question 1

empirical formula

Answer 1

this is the simplest whole number ratio of atoms or ions of each element
—> simplest ratio or moles of atoms in a compound
e.g. C3H6 / 3 = CH2 (simplest ratio)

Question 2

molecular formula

Answer 2

the actual number of atoms of each element in a compound
—> the ratio of moles of atoms in a compound
e.g. C3H6 = 3 moles of carbon, 6 moles of hydrogen

Question 3

relative formula mass (RFM)

Answer 3

sum of the relative atomic masses or all the atoms or ions in its formula

Question 4

equation for % composition

Answer 4

mass of the element / mass of the compound x100

Question 5

avogadro constant number

Answer 5

6.02 x 10(to the power of 23)

Question 6

how to work out number of molecules/atoms

Answer 6

avogadro number x moles

Question 7

mole

Answer 7

an amount of a substance

Question 8

avogadro constant equation

Answer 8

number of atoms/number of molecules
moles

Question 9

how to work out moles

Answer 9

moles = mass
RAM/RFM

Question 10

conservation of mass

Answer 10

mass cannot be created or destroyed

Question 11

how to work out mole ratio

Answer 11

divide both moles of the element by the smallest number of moles to obtain the simplest ratio

Question 12

how to work out empirical formula by using mass

Answer 12

1) calcule the number of moles in each element
2) determine the mole ratio between the elements

e.g. 2.80g or iron combined with 5.33g of chlorine
1) Fe Cl
2.8g = 0.05 5.33g = 0.15
56 35.5

2) 0.05 = 1 0.15 = 3
0.05 0.05

                    Fe:Cl
                      1:3

Question 13

how do you work out reacting mass calculations

Answer 13

1) write the ratio of moles for each substance
2) write✔️ for the substance whose mass is given and ? for the substance who’s mass is to be calculated on the balanced equation
3) find the moles of the ✔️ substance
4) use the balanced equation (ratio of moles) and your answer from step 2 is to find the moles of the ? substance
5) find the mass or the ? substance

e.g. what mass of aluminium is needed for react with 640g or iron oxide Fe2O3 + 2(Al) —> 2(Fe) + Al2O3

1) Fe2O3 + 2(Al) —> 2(Fe) + Al2O3
1 : 2 —> 2 : 1
2) ✔️ ?

3) 640 = 4mol
160

4) because of 1:2 ratio
4 x 2 = 8mol

5) 8mol x 27 = 216g

Question 14

how to work out mass

Answer 14

mass = mol x RFM

Question 15

concentration equation

Answer 15

mass of solute (g)
volume of solution (dm3)