1 25cm3 of sodium hydroxide (NaOH) at 1.5 mol/dm3 neutralises 21.5cm3 of
hydrochloric acid (HCl). Find the concentration of the acid in moles per
decimetre cubed (mol/dm3). Decimetres cubed is the same thing as litres
1.1 So we know that in 1 litre of NaOH, we've got 1.5
moles. How much do we have in one millilitre?
1.1.1 1ml = 15/1000 = 0.0015 moles per ml (because there are
1000 millilitres in a litre, we divide by 1000.)
188.8.131.52 But we don't have 1 ml of it, we have 25 ml of it, so we multiply by 25.
25 x 0.00515 = 0.0375 moles. Now, this tells us how much stuff we've got
in each chemical.
184.108.40.206.1 Because there are the same number of moles in each chemical. If there weren't, and
the ratios were different, we would simply multiply or divide as needed.
220.127.116.11.2 But, again, we don't have equal amounts of each substance, so we need to first find out the
concentration of 1 millilitre (cm3) of HCl. 0.0375/21.5 (because that's how much HCl we've
got) = 0.0017441 moles in a cm3 of HCl.
18.104.22.168.2.1 The question is asking for the concentration of the HCl in moles per decimetre cubed, so we
multiply by 1000, because that's how many millilitres we've got in a litre. 0.0017441 x 1000 = 1.74
1.2 First, we need to balance the equation: NaCl + HCl ==> NaCl + H2O
1.2.1 From this, we can see that the ration of sodium hydroxide to hydrochloric acid is 1:1, so
for each mole of NaCl, there is 1 mole of HCl.