O-LevelChemistryStoichiometry5070/22 · Oct/Nov 2023 · Q711 Marks

Dilute hydrochloric acid reacts with aqueous sodium carbonate. 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂ A student titrates 20.0cm³ of 0.0250mol/dm³ aqueous sodium carbonate with dilute hydrochloric acid using methyl orange as an indicator. A volume of 15.5cm³ of dilute hydrochloric acid reacts exactly with the 0.0250mol/dm³ aqueous sodium carbonate. Calculate the concentration, in mol/dm³, of the dilute hydrochloric acid. concentration of dilute hydrochloric acid _________________________ mol/dm³

(a)Dilute hydrochloric acid reacts with aqueous sodium carbonate. 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂ A student titrates 20.0cm³ of 0.0250mol/dm³ aqueous sodium carbonate with dilute hydrochloric acid using methyl orange as an indicator. A volume of 15.5cm³ of dilute hydrochloric acid reacts exactly with the 0.0250mol/dm³ aqueous sodium carbonate. Calculate the concentration, in mol/dm³, of the dilute hydrochloric acid. concentration of dilute hydrochloric acid _________________________ mol/dm³[3]
(b)(i)State the colour of methyl orange in alkaline solution.[1]
(b)(ii)Write the formula of the ion present in aqueous solutions of alkalis.[1]
(c)Calculate the volume, measured at r.t.p., of carbon dioxide produced, in cm³, when 2.65g of sodium carbonate reacts with excess hydrochloric acid. 2HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂ volume of carbon dioxide _________________________________ cm³[2]
(d)Hydrochloric acid is a strong acid. Define the term strong in the phrase strong acid.[1]
(e)Oxides of nitrogen contribute to acid rain.
(e)(i)State one other adverse effect of oxides of nitrogen.[1]
(e)(ii)Oxides of nitrogen are removed from car exhausts by catalytic converters. Complete the symbol equation for the reaction that occurs in catalytic converters. 2CO + 2NO → ____________________ + ____________________[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2023). The mark or mark type is shown in brackets.

[1] mol Na2CO3 = 20.00/1000 × 0.025 OR 5 × 10-4 (1)
[1] mol HCl = 1 × 10-3 (1)
[1] (concentration of HCl =) 0.0645 (mol / dm³) (1)
[1] yellow
[1] OH-
[1] mol Na2CO3 = 2.65/106 OR 0.025 (1)
[1] volume of CO2 = 600 cm³
[1] (acid which is) completely dissociated / completely ionised (in aqueous solution)
[1] (photochemical) smog / respiratory problems
[1] N2 (1)
[1] 2CO2 (1)

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