3.1.2 Amount of Substance

Define what is meant by the follwoing:

• relative atomic mass

• relative molecular mass

• relative atomic mass - the average mass of an atom of an element relative to 1/12 the mass of an atom of carbon-12

• relative molecular mass - the average mass of a molecule relative to 1/12 the mass of an atom of carbon-12

Explain what Avogadro’s constant shows us about the mole

the mole is the amount of substance in grams that has the same number of particles as there are atoms in 12g of carbon 12

Write the equation to work out the number of entities

No of particles = moles of substance (in mol) X Avogadro's constant

State how to work out moles given mass and Ar or Mr

moles = mass moles = mass

Ar Mr

State how would you work out moles given concentration and volume include units

moles = concentration ( mol dm^-3 ) x volume (dm³)

State how would you convert the following

• from cm³ to dm³

• from mg to g

• from kg to tonne

• from g to kg

• from ^oC to kelvin

cm3 —→ dm3 = divide by 1000

mg —→ g = divide by 1000

kg —→ tonne = divide by 1000

g —→ kg = divide by 1000

^oC —→ kelvine = add 273

What is the number of moles in 75.0mg of CaSO₄ .2H₂O?

75mg —→ 0.075g

moles = mass / mr

Mr = 40 + 32 + 64 + (2 × 18) = 172

moles = 0.075 / 172 = 0.000436 mol

= 4.36 × 10^-4 mol

Calculate the number of atoms of tin in a 6.00 g sample of tin metal.

Calculate the number of chloride ions in a 25.0 cm³ of a solution of magnesium chloride of concentration 0.400 mol dm^-3

Write down the ideal gas equation include units

PV = nRT

• P = pressure - Pa

• V = volume - m³

• n = moles - mol

• R = 8.31

• T = temperature - K (kelvin)

Write down the equation that links:

• Volume ₁

• Volume ₂

• Pressure ₁

• Pressure ₂

• Temperature ₁

• Temperature ₂

Write down the equation that links:

• Volume ₁

• Volume ₂

• Pressure ₁

• Pressure ₂

• Temperature ₁

• Temperature ₂

A gas has an initial volume of 300m³ at standard atmospheric pressure (100kPa). Calculate the final volume of this gas if it’s pressure is increased by 400kPa at a constant temperature.

boyle’s law: P₁ x V₁ = P₂ x V₂

100kPa x 300 m³ = 500kPa x V₂

V₂ = 300,000 / 500 = 60 m³

Calculate the mass of Cl₂ gas that has a pressure of 100 kPa, temperature 20 ^oC , volume 500 cm³ . (R = 8.31)

Write the equation to work out density

density ( g cm³ ) = mass (g)

volume (cm³)

Calculate the number of molecules of ethanol in a 0.500 dm³ of ethanol (CH₃CH₂OH) liquid. The density of ethanol is 0.789 g cm^-3

There are 980 mol of pure gold in a bar measuring 10 cm by 20 cm by 50 cm. Calculate the density of gold in kg dm^-3

Define the following terms

• emperical formula

• molecula formula

• emperical formula - the simplest whole number ratio of atoms of each element in a compound

• molecular formula - the actual number of atoms of each element in a compound

A compound contains 0.34g of carbon, 0.057g of hydrogen and 0.46g of oxygen.

Find the empirical formula and hence the molecular formula

(Mr = 180)

What is meant by water of crystallisation?

• Water of crystallisation is when some compounds can form crystals which have water as part of their structure

• A compound that contains water of crystallisation is called a hydrated compound

• a compound that does not contain water is called an anyhydrous compound

3.120g of hydrated magnesium sulfate MgSO₄ .xH₂O was heated to make 1.567g of anyhydrous magnesium sulfate. Find the vlue of x and hence give the formula of the hydrated salt.

0.58g of compound X containing only carbon, hydrogen and oxygen, gave 1.32g of carbon dioxide and 0.54g of water on complete combustion in oxygen.

What is it’s empirical formula?

What is it’s molecular formula if it’s relative molecular mass is 58.0?