Prelab Assignment Experiment 2 Answers
1. What is the molar mass of xylenesulphonic acid dihydrate?
222 g mol-1
Just add up the atomic masses of the component atoms. If you haven't got
the right answer you've probably left out the two water molecules. These are part of the
material when it crystallizes, so must be taken into account when we determine the MW.
2. How many moles of hydrogen ions are liberated when 0.4673 mol of xylenesulphonic acid is
dissolved in water?
0.4673 moles
The equation in the manual on page 15 tells us that 1 mole of acid
generates one mole of hydrogen ions, so the calculation is trivial.
Note that (a) since a single right-facing arrow is drawn, the equation implies that
xylenesulphonic acid is a strong acid (fully dissociated in solution), which is why we know
that 0.4673 moles of acid generate the same quantity of hydrogen ions; but (b) strictly the
arrow in the equation should be an equilibrium sign, since, although the position of
equilibrium lies strongly to the right, the dissociation is reversible and the undissociated acid
is re-formed if the solution is evaporated.
3. If 0.4015g of potassium hydrogen phthalate required 28.65 ml of sodium hydroxide solution
to neutralize it, what is the concentration of sodium hydroxide? Note that potassium
hydrogen phthalate has one acidic hydrogen and reacts 1:1 with NaOH.
(i) how many moles of acid are there? (ii) how much base reacts with the acid? (iii) how
many moles of base are there in a litre of solution?
(i) Moles of acid = weight/MW = 0.4215/204 = 1.968 x 10-3 mol
(ii) Acid-base reaction is 1:1, so moles of base = 1.968 x 10-3 mol
(iii) Moles of base per litre = 1.968 x 10-3 mol x 103 ml l-1 / 28.65 ml = 0.0689 mol l-1 = molarity
of the solution
4. If 0.1245g of an unknown acid required 24.78 ml of the sodium hydroxide solution used in
question 3 to neutralize it, how many grams of acid are needed to neutralize one mole of
sodium hydroxide?
(Note that we don't know from the question how many hydrogens the unknown acid has per
molecule, and it might seem as though we will need that; we could assume that the acid is
monoprotic, in other words, that it has only one acid proton per molecule. In fact though, it
doesn’t matter whether the acid is monoprotic or not – all we need to know is how many
moles of base are in the sodium hydroxide solution; from this we can calculate the amount
of solid acid required.)
(i) No. of moles of base = 24.78 ml x 0.689 mol l-1 / 1000 ml l-1 = 1.707 x 10-3 mol
(ii) Since the acid base reaction is 1:1, this is also the number of moles of acid
(iii) Using moles = weight/MW, weight = 0.1245 / 1.707 x 10-3 = 72.9 g.