This medicine is supplied as a concentrated potassium chloride solution for hospital use. Fig. 1 shows an ampoule containing 10.0 cm3 of solution. The label...
Question 1 Report
This medicine is supplied as a concentrated potassium chloride solution for hospital use. Fig. 1 shows an ampoule containing 10.0 cm3 of solution. The label states that the concentration is 2.00 mol/dm3. A nurse adds the full ampoule to a saline bag and makes the total volume 500 cm3.
(a) Give the number of moles of potassium chloride in the ampoule. [2]
(b) Use the final volume to calculate the concentration after dilution. [2]
(c) Suggest why a concentrated ampoule must not be used without dilution. [1]
Fig. 1 shows a soil scientist extracting nitrate ions from 25.0 cm3 of soil water. The extracted sample is titrated with 0.0400 mol/dm3 iron(II) solution. The mean titre is 12.50 cm3. One mole of nitrate ions reacts with three moles of iron(II) ions in the method used.
(a) Give the number of moles of iron(II) ions in the mean titre. [2]
(b) Use the ratio to calculate the number of moles of nitrate ions in the sample. [1]
(c) What is the nitrate concentration in the soil water in mol/dm3? [2]
Answer Details
Potassium chloride dilution
- (a) Convert \(10.0\text{ cm}^3\) to \(0.0100\text{ dm}^3\). Then use \(n=cV\): \[n=2.00\times0.0100=0.0200\text{ mol}\] [2 marks]
- (b) The final volume is \(500\text{ cm}^3=0.500\text{ dm}^3\). The amount of KCl does not change during dilution, so \[c=\frac{0.0200}{0.500}=0.0400\text{ mol/dm}^3\] [2 marks]
- (c) The concentrated ampoule would give an unsafe, excessively concentrated dose if used without dilution. [1 mark]
Nitrate titration
- (a) Convert the mean titre: \(12.50\text{ cm}^3=0.01250\text{ dm}^3\). \[n(\text{Fe}^{2+})=cV=0.0400\times0.01250=0.000500\text{ mol}\] [2 marks]
- (b) One mole of nitrate reacts with three moles of iron(II) ions. Therefore \[n(\text{nitrate})=\frac{0.000500}{3}=0.0001667\text{ mol}\] [1 mark]
- (c) The soil-water sample volume is \(25.0\text{ cm}^3=0.0250\text{ dm}^3\). \[c(\text{nitrate})=\frac{0.0001667}{0.0250}=0.00667\text{ mol/dm}^3\] [2 marks]
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