O-LevelChemistryStoichiometry5070/21 · Oct/Nov 2020 · Q810 Marks

Dilute nitric acid reacts with aqueous barium hydroxide. 2HNO₃(aq) + Ba(OH)₂(aq) → Ba(NO₃)₂(aq) + 2H₂O(l)

(a)(i)A student titrates 25.0cm³ of dilute nitric acid with 0.0450mol/dm³ barium hydroxide using methyl orange as an indicator. A volume of 34.0cm³ of aqueous barium hydroxide reacts exactly with the dilute nitric acid. Calculate the concentration of the dilute nitric acid. concentration of nitric acid ................................................ mol/dm³[3]
(a)(ii)Describe how to prepare pure dry crystals of barium nitrate from aqueous barium nitrate.[3]
(b)Barium nitrate decomposes when heated to form barium oxide, BaO, nitrogen dioxide, NO₂, and oxygen. Construct the equation for this reaction.[1]
(c)Nitrogen dioxide is an atmospheric pollutant which causes 'acid rain'. Describe one effect of acid rain on buildings.[1]
(d)(i)Hydrazine, H₂N – NH₂, is a colourless liquid. Draw a dot-and-cross diagram for a molecule of hydrazine. Include only the outer shell electrons.[1]
(d)(ii)The melting point of hydrazine is 2°C. The boiling point of hydrazine is 114°C. Use this information to suggest why hydrazine is a solid at 0°C.[1]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

[3] 8(a)(i) mol barium hydroxide = 0.045 × 34.0/1000 OR 1.53 x 10-3 (1) mol nitric acid = 3.06 × 10-3 (1) concentration nitric acid = 0.122(4) mol/dm³ (1)
[3] 8(a)(ii) evaporate solution until crystallisation point / evaporate until solution is saturated (1) filter off crystals AND wash with organic solvent / wash with cold water (1) dry crystals with filter paper (1)
[1] 8(b) 2Ba(NO3)2 → 2BaO + 4NO2 + O2
[1] 8(c) erodes (buildings) / corrodes (metalwork) / reacts with (mortar)
[1] 8(d)(i) H H :N:N: H H
[1] 8(d)(ii) 0°C is below the melting point / the melting point is above 0°C

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