Quantitative Chemistry

Calculations in Chemistry: part 2

Compound Formation

A particular compound always contains the same elements.

Independent of how you make the compound the elements are always present in the same proportion by mass.

These proportions cannot be changed.

Magnesium oxide always contains 60% Mg and 40% O by mass.

Reacting amounts of substance

Relative formula masses can also be used to calculate the amounts of compounds reacted together or produced in reactions.

If 0.24g of Mg react with 0.16g of O to produce 0.40g of MgO.......... How much MgO will be produced by burning 12g of Mg?

0.24g Mg produces 0.40g of MgO

so 1g of Mg produces 0.40/0.24 g of MgO= 1.67 g of MgO

so 12g of Mg produces 12 x 1.67 g MgO

= 20g of MgO

Important Calculations of quantities like these are a very important part of chemistry.

There is a great deal of information stored in the formulae and the equation.

ActivityQuestion 3 on page 181

Extension: Read through pg 178 – 179 (compound formation and chamical formulea.

Answer question 4 on page 181

The Mole• When carrying out an

experiment a chemist can not weigh out single atoms or molecules.

• A counting unit was found.• The standard unit of a substance

is the relative atomic mass in grams.

• This unit is called 1 mole (1 mol)• Carbon Ar = 12• 1 mole of carbon is 12g

In a similar way banks weigh

coins because they know how much one coin

weighs

Avogadro‘s Constant (L)

1 mole of any substance contains the same number of atoms.

6.02 x 1023 atoms per mole6.02 x 1023 coke

cans stacked together would

cover the surface of the Earth to a

depth of 200 miles.

Calcualtions involving the

MoleHow to calculate the molar mass.

Write the formula for ethanol C2H5OHCalcualte Mr = (2x12) + (5 x 1) + 16 + 1

= 46• The molar mass of ethanol is

46g/mol

CalculationsFor any given mass of a substance you can calculate the moles present.

Mass

Number Mr Of Moles

ExampleHow many moles are there in 60g of Sodium Hydroxide?

Mr of NaOH = 23 + 16 + 1 = 40

Molar mass of NaOH = 40g /mol

Number of moles = mass / molar mass

= 60 / 40

= 1.5 mols

ActivityWorksheet 6.2

Working out the Empirical Formula

We can work out the chemical formula from experimental data.

Magnesium reacts with oxygen to form magnesium oxide. In an experiment if we know that 0.24g of Mg reacts with 0.16g of O2 we can determine the formula. Mg O

Mass combined o.24 g 0.16gMolar mass 24 g/mol 16 g/molNumber of moles

0.01 mol 0.01 mol

Simplest ratio 1 1formula MgO

Activity: Calculate the empirical formula for silicon oxide

Si OMass 47g 53gMolar massNumber of molesSimplest rationFormula

Complete questions on page 185

A Chemical Footbridge: calculating reacting amounts

What mass of aluminium oxide is produced when 9.2g of aluminuim metal react with oxygen?

Balanced equation:4Al + 3O2 2Al2O3 Ratio 4:2

Convert 9.2 g of Al into moles.Number of moles = mass/ molar mass = 9.2 / 27 = 0.34 mol

Use the ratio from the equation to work out how many moles of Al2O3 are produced.

4 mol of Al produce 2 mol of Al2O3So 0.34 mol of Al produce 0.17 mol of Al2O3

A Chemical Footbridge: calculating reacting

amounts …..continuedWork out the mass of Al2O3

Mr = 102Mass = molar mass x number of moles

= 102 x 0.17 = 17.3 g

Concentrations of Solutions

Two types of concentrationMass concentration - g/dm³Molar concentration - mol / dm³

1dm³ = 1000 cm³ = 10 cm x 10 cm x 10cm = 1 Litre

Concetration = mass of solute

volume of solution

ConcentrationA 1 mol /dm³ solution of sodium chloride contains 58.5g of NaCl (1 mol) dissolved in water and made up to a final volume of 1 dm³ or 1 litre.

Concentration calculations

How many moles of suger are there in 500 cm³ of a 3.0 mol/dm³ sugar solution?

Number of moles = concentration x volume

1000

= 3.0/ 1000 x 500

= 1.5 mol

ExampleCalculate the concentration of a solution of sodium hydrioxide , NaOH, that contains 10g of NaOH in a final volume of 250 cm³ (= 0.25 dm³)

Calculate number of molesMr = 23 + 16 + 1 = 40Number of moles = mass/ Mr = 10/40 = 0.25 mol

Find the concentrationNumber of moles = concentration x volumeConcentration = number of moles / volume

= 0.25 / 0.25 = 1 mol /dm³

ActivityRead pages 192 and 193