Concentration in grams and in moles per cubic decimetre, the titration method and calculations, and using the molar gas volume. Chemistry only content.
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1.What does concentration measure?
How much solute is dissolved in a given volume of solution.
2.State the equation for concentration in grams per cubic decimetre.
concentration = mass of solute in grams divided by volume in cubic decimetres.
3.How many cubic centimetres are in one cubic decimetre?
1000.
4.Convert 250 cm3 into dm3.
250 divided by 1000 = 0.25 dm3.
5.10 g of solute is dissolved in 500 cm3. Calculate the concentration in g/dm3.
500 cm3 = 0.5 dm3. 10 divided by 0.5 = 20 g/dm3.
6.State the equation for concentration in moles per cubic decimetre.
concentration in mol/dm3 = moles of solute divided by volume in dm3.
7.Rearrange to find moles.
moles = concentration x volume in dm3
8.Calculate the moles in 25 cm3 of a 0.1 mol/dm3 solution.
25 cm3 = 0.025 dm3. 0.1 x 0.025 = 0.0025 moles.
9.How do you convert g/dm3 into mol/dm3?
Divide the concentration in g/dm3 by the relative formula mass.
10.A solution is 40 g/dm3 of NaOH, Mr 40. What is its concentration in mol/dm3?
40 divided by 40 = 1 mol/dm3.
11.What does a titration allow you to find?
The volume of one solution needed to react exactly with a known volume of another, and from that an unknown concentration.
12.Which piece of apparatus measures the solution being added?
A burette.
13.Which piece of apparatus measures the fixed volume placed in the flask?
A pipette.
14.Why is a pipette used rather than a measuring cylinder?
It measures a fixed volume much more accurately.
15.What is the end point of a titration?
The moment the indicator changes colour, showing the reaction is exactly complete.
16.Name two indicators suitable for titrations.
Phenolphthalein and methyl orange.
17.What colour change does phenolphthalein show going from alkali to acid?
Pink to colourless.
18.What colour change does methyl orange show going from alkali to acid?
Yellow to red.
19.Why is universal indicator not used for titrations?
It changes colour gradually through a range of colours, so there is no sharp end point.
20.What are concordant results?
Repeat titres that are within 0.10 cm3 of each other.
21.Why is a rough titration done first?
To find the approximate end point, so the accurate runs can be added slowly near that volume.
22.Which results are used to calculate the mean titre?
Only the concordant results. The rough titration is excluded.
23.What is the volume of one mole of any gas at room temperature and pressure?
24 dm3, or 24000 cm3.
24.State the equation linking gas volume and moles.
volume in dm3 = moles x 24
25.What is the volume of 2 moles of carbon dioxide at room temperature and pressure?
2 x 24 = 48 dm3.
26.How many moles are in 12 dm3 of a gas at room temperature and pressure?
12 divided by 24 = 0.5 moles.
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