This question was attempted by most candidates and the performance was good.
In part (a) candidates correctly wrote balanced chemical equation for the reaction thus:
HNO3(aq) + NaOH(aq) → NaNO3(aq) H2O(l)
In (b)(i) –(iv), majority of the candidates were able to calculate the concentration of NaOH(aq) in moldm-3 concentration of HNO3 in moldm-3 and gdm-3 respectively. However, some of them could not correctly determine the number of hydrogen ions in gdm-3 of HNO3 nor express their values in the correct number of significant figures.
The required answer from candidates were
i) Molar mass of NaOH = 23 + 16 + 1 or 40gmol-1
Mass concentration of NaOH = 8.00gdm-3
Concentration of NaOH = 8.00gdm-3
40gmol-1
= 0.20moldm-3
[Any other correct method was accepted]
(ii) Concentration of HNO3 in moldm-3
CA VA = 1
CB VB 1
CA = CB x VB
VA
= 0.20 x 20
22.60
= 0.177 mol dm-3
(iii) Mass concentration of HNO3 in gdm-3
Molar mass of HNO3 = 1 + 14 + 16 x 3 or 63 gmol-1 (correct unit)
Mass conc. of HNO3 = 0.177 moldm-3 x 63 gmol-1
= 11.15 gdm-3
(iv) 1 mole of HNO3 ≡1 mole of H+ ions or
0.177 mole of HNO3 ≡ 0.177 mole of H+ ions
number of H+ in 1dm3 of HNO3 = 6.02 x 1023 x 0.177
= 1.066 x 1023ions
