You have read the exam question carefully and decided that the following are the key bits of information: 3.70 g of insoluble MCO3 is added to 250 cm3 of 0.5 mol HNO3. A 20 cm3 sample of the solution is titrated with 19.9 cm3 of 0.3 mol dm-3 NaOH. Calculate the relative atomic mass of M,…
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Back titration to determine the amount of an insoluble base
You have £20.00 in your pocket. You go to the shops and buy a few things. When you get home, you have £3.73 left. How much did you spend? You hopefully said £16.27. A back titration is essentially the same as the calculation you’ve just done – by determining how much of something was left…
Titration to calculate the purity of a solid
You have read the exam question carefully and decided that the following are the key bits of information: 1.50 g of impure Na2O reacts and dissolves to make a 150 cm3 solution A 50 cm3 sample is titrated with 13.7 cm3 of 0.5 mol dm-3 HNO3 Determine the % by mass purity of Na2O. Step 1 We…
Titration to calculate the water of crystallisation
You have read the exam question carefully and decided that the following are the key bits of information: 3.69 g of Ba(OH)2.xH2O is made into a 250 cm3 solution A 25 cm3 sample of the solution is titrated with 23.4 cm3 of 0.1 mol/dm3 HCl. Calculate the relative formula mass of Ba(OH)2.xH2O, and hence x.…
Titration to identify the element M
You have read the exam question carefully and decided that the following are the key bits of information: 3.69 g of soluble M2CO3 is made into a 250 cm3 solution A 25 cm3 sample of the solution is titrated with 26.7 cm3 of 0.2 mol dm-3 HNO3. Calculate the relative atomic mass of M, and hence, identify…
Titration to calculate the concentration of acids and alkalis
You will meet a very wide variety of titrations on the A Level Chemistry course. Titration calculations can sometimes be quite complex (and worth 6, 7 or 8 marks!) but the basic procedure will always be the same: First, use the data in the question to find the moles of one substance. Then, use the…
Big Idea #5: Exam Technique
Exam questions on chemical analyses (especially titrations) are designed to make you think and test whether you can pull out the key bits of information relevant to a calculation. There will always be lots of detail that you can safely ignore and even very long questions could probably be condensed down to a few bullet…
Big Idea #4: Redox equations
Many reactions in Chemistry are redox reactions. This means that reduction and oxidation reactions are occuring at the same time, with one substance gaining electrons (reduction) and the other losing electrons (oxidation). You need to be able to work out the balanced symbol equation for a redox reaction, usually starting from with a few bits…
Big Idea #3: Using ICE tables
A balanced symbol equation tells us the ratio between the changes in the number of moles of each substance in a reaction. If: 2 C2H6 + 7 O2 –> 4 CO2 + 6 H2O Then: the moles of C2H6, O2, CO2 and H2O change in a 2:7:4:6 ratio. A very common type of question in Chemistry is where we need to calculate the amount of each product formed from…
Big Idea #2: Mole calculations
There are several different ways of calculating moles and you need to be confident with all of them. You’ll usually have to use more than one approach to calculating moles in any given question. I’ve worked through some very straightforward examples to help you practice with each of the equations and ideas. The examples only…