CHEMICAL FORMULAE, EQUATIONS AND CALCULATIONS: PART 2 [dchemlc]

what is avagadro's law?

• Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules

• Equal masses of gases at the same temperature and pressure contain equal numbers of molecules

• Equal volumes of gases at the constant temperature and pressure contain boyle's number of molecules

• Equal volumes of gases at the same temperature and pressure contain different numbers of molecules

Answer explanation

Avogadro's law:

Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

which of the following is not a unit of volume?

• dm

• cm³

• dm³

• l

Answer explanation

Volumes (of gases or liquids) are measured in

#cubic centimetres (cm3) or #cubic decimetres (dm3) or #litres (I)

1 litre= 1 dm3 = 1000cm3

The volume occupied by 1 mole of a gas is often called the molar volume. What is the molar volume of a gas?

• 24dm³

• 24cm³

• 24l

• 42dm³

Answer explanation

At room temperature and pressure, the volume occupied by 1 mole of any gas is approximately 24 dm³

(24 000 cm³).

The volume occupied by 1 mole of a gas is often called the molar volume.

_________ is a technique that is used to find out how much of one solution reacts with a certain volume of another solution of known concentration.

Calculate the number of moles of NaOH in 0.050dm³ of 0.10mol/dm³ solution.

Answer explanation

Number of moles = volume of solution (dm³) x concentration (mol/dm³). Number of moles= 0.050 x0.10 = 0.0050mol.

which of the following is not a unit of concentration?

• mol/cm

• g/dm³

• mol/dm³

• mol/l

Answer explanation

Concentrations can be measured in either

• g/dm3

or

• mol/dm3.

These can also be written as g dm^-3 and mol dm^-3.

You read them as 'grams per cubic decimetre' and 'moles per cubic decimetre'. Remember: 1 cubic decimetre is the same as 1 litre.

convert 75cm³ into dm³.

• 0.075dm³

• 0.75dm³

• 75dm³

• 75000dm³

Answer explanation

Because 1 dm3 is equivalent to 1000cm', to convert dm3 to cm' we have to go from 1 to 1000, therefore we need to multiply by 1000.

To convert cm' to dm3 we need to go from 1000 down to 1, therefore we divide by 1000.

What is the concentration of a 0.050 mol/dm³ solution of sodium carbonate, Na₂CO₃, in g/dm³?

1 mol of Na₂CO₃ weighs 106g.

• 5.3g/dm³

• 53g/dm³

• 0.050g/dm³

• 50g/dm³

Answer explanation

1 dm³ of solution contains 0.050 mol of Na₂CO₃.

1 mol of Na₂CO₃ weighs 106g. 0.050 mol weighs

0.050 x 106 = 5.3g.

1 dm³ of solution contains 5.3g Na₂CO₃, therefore the concentration is 5.3g/dm³.

Calculate the volume in dm³ of 0.20 mol H₂O at room temperature and pressure.

• 4.8 dm³

• 4.8 cm³

• 48 dm³

• 24 dm³

Answer explanation

volume = number of moles x molar volume

Because we want the volume in dm³ we use 24 dm³ as the molar volume:

volume= 0.20 x 24 = 4.8 dm³

which of the following equations can be used to find the number of moles in a given volume?

• 
• 
• 
• 

Answer explanation