This diagram is of a water-treatment tank where sodium carbonate solution is added to acidic water. The operator has 2.50 dm3 of sodium carbonate solution a...
Question 1 Report
This diagram is of a water-treatment tank where sodium carbonate solution is added to acidic water. The operator has 2.50 dm3 of sodium carbonate solution at a concentration of 0.0800 mol dm-3. The tank display records the volume pumped. The carbonate reacts with acid and can form carbon dioxide gas.
(a) Use the concentration and volume to calculate the amount of sodium carbonate pumped into the tank. [2]
(b) Give the volume in cm3 shown by 2.50 dm3. [1]
(c) Describe one observation if excess acid reacts with carbonate in the tank. [2]
Fig. 1 shows a student warming a solid sample of hydrated sodium carbonate before preparing a solution. The student should not heat the compound in this method because the measured mass is needed for an accurate molar concentration. The student dissolves 14.3 g of Na2CO3.10H2O and makes 500 cm3 of solution. Its relative formula mass is 286.
(a) Use the mass and relative formula mass to calculate the amount of hydrated sodium carbonate. [2]
(b) Give the concentration of the solution in mol dm-3. [2]
(c) Describe why heating the hydrated solid before weighing would give an unreliable concentration. [1]
Answer Details
Sodium carbonate treatment solution
(a) Use \(n=cV\):
\[n=0.0800\times2.50=0.200\text{ mol}\]
The amount pumped into the tank is 0.200 mol of sodium carbonate. [2 marks]
(b)
\[2.50\text{ dm}^3=2500\text{ cm}^3\]
The displayed volume is 2500 cm3. [1 mark]
(c) Bubbles or effervescence would be observed because carbon dioxide gas is produced when excess acid reacts with carbonate. [2 marks]
Hydrated sodium carbonate solution
(a)
\[n=\frac{m}{M_r}=\frac{14.3}{286}=0.0500\text{ mol}\]
The amount of hydrated sodium carbonate is 0.0500 mol. [2 marks]
(b)
\[500\text{ cm}^3=0.500\text{ dm}^3\]
\[c=\frac{0.0500}{0.500}=0.100\text{ mol dm}^{-3}\]
The concentration is 0.100 mol dm-3. [2 marks]
(c) Heating can remove water of crystallisation, changing the mass and composition of the hydrated solid. The amount calculated from the original formula mass would then be unreliable. [1 mark]
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