# Formulas

Relative formula mass, % mass etc.

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• Created by: Kavita :)
• Created on: 15-01-11 15:02

## Relative Formula Mass

To find the relative formula mass (Mr) of a compound, add together the Ar values for all the atoms in its formula.

• Find the Mr of carbon monoxide (CO).

C = 12

O = 16

12 + 16 = 28

• Find the Mr of sodium oxide (Na2O)

Na = 23

O = 16

(23 x 2) + 16 = 62

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## Calculating % Mass of An Element in a compound

Percentage mass of element =

Total mass of element / Total mass of compound x 100

• What is the percentage  mass of oxygen (O) in sodium hydroxide (NaOH)?

Mr of NaOH is 23 + 16 + 1 = 40

(16 / 40 ) × 100 = 0.4 × 100 = 40%

• What is the percentage mass of carbon in zinc carbonate (ZnCO3)?

M of ZnCO3 is 65 + 12 + (3 x 16)

(12 / 126) x 100 = 9.6 %

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## Empirical Formula

1) List all the elements in the compound

2) Write experimental masses or percentages

3) Divide them all by Relative Atomic Mass of element

4) Simplify (ratio)

Suppose 3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?  (sulfur is 32  oxygen is 16)

sulfur: 3.2 ÷ 32 = 0.1

oxygen: 3.2 ÷ 16 = 0.2              Ratio  0.1 : 0.2      ( x 10)     Ratio   1 : 2

Formula: SO2

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## Calculating masses in reactions

1) Write out the balanced equation

2) Work out Mr - just for the bits you need

3) Divide to get ONE, then mutiply to get ALL

What mass of magnesium oxide is produced when 60g of magnesium is burned in air?

1)    2Mg + O2 ----->   2MgO

2) 2 x 24 ------> 2 x (24 +16)    (ignore oxygen)

48     ---->           80

3) 48 of Mg reacts to give 80g of MgO

1g of Mg reacts to give 1.67g of MgO

60g of Mg reacts to give 100g of MgO

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## Calculating Masses in reactions

How much Hydrochloric acid do we need to neutralise 50kg of magnesium carbonate?

1) 2HCl + MgCO3 ----> MgCO2 + CO2 + H2O

2) H = (2 x 1) = 2  Cl = (2 x 35) = 70  total: 72

Mg = (1 x 24) = 24 C = (1 x 12) = 12 O3 = (3 x 16) = 48  total: 84

(72 / 84) x 50 = 42.9

42.9kg of hydrochloric acid

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## The Mole

One mole of atoms of molecules of any substance will have the mass in grams equal to the relative formula mass for that substance.

Number of moles = Mass in g (of element/compound) / Mr (of element/compound)

e.g

How many moles are there in 42g of Carbon?

No. of moles = 42/12 = 3.5 moles

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## Atom Economy

% of reactants changed to useful products

What is the atom economy for making hydrogen by reacting coal with steam?

C(s) + 2H2O(g)    →    CO2(g) + 2H2(g)

12       2 × 18 = 36               44           2 × 2 = 4

total mass of products = 44 + 4 = 48g

% atom economy = 448 × 100 = 8.3%

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## Percentage Yield

Compares Actual and Predicted yield.

Yields are always less than 100 %

Calculate the maximum theoretical yield when 3.2 kg of methane reacts with oxygen to make water.

CH4 + 2O2 -> CO2 + 2H2O

CH4 = 16     16/5 = 3.2

2O2 = 64

64/5 =12.8             3.2 + 12.8 = 16 - theoretical yield

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## Percentage Yield

Calculate the percentage yield if 5.8 kg is obtained.

2H2O = 36

36/5 = 7.2

3.2 + 12.8 -> 8.8 +7.2

(5.8 / 7.2) x 100 = 80.6 %  - water

(5.8 / 16) x 100 = 36.25 %

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