The table below shows results from an investigation into the dilution of ammonia solution used in a window-cleaning product. Fig. 1 shows a pipette transfer...
Question 1 Report
The table below shows results from an investigation into the dilution of ammonia solution used in a window-cleaning product. Fig. 1 shows a pipette transferring 10.0 cm3 of stock solution to a flask, which is then filled to 100 cm3 with water.
| Solution | Volume of ammonia solution / cm3 | Final volume / cm3 |
|---|---|---|
| Stock | 10.0 | 10.0 |
| Diluted | 10.0 | 100 |
The stock solution has concentration 2.00 mol/dm3.
(a) Complete the statement: dilution changes the volume but not the number of moles of ammonia transferred. [1]
(b) Use the volumes to calculate the concentration of the diluted solution. [2]
(c) Suggest why a pipette is used instead of a measuring cylinder. [1]
Fig. 1 shows a student preparing 250 cm3 of sodium chloride solution for a conductivity test. A balance reading shows that 2.92 g of sodium chloride, NaCl, was transferred to a volumetric flask. The student adds water, swirls until the compound dissolves, then makes the solution up to the mark.
The relative formula mass, Mr, of NaCl is 58.5.
(a) Complete the calculation of the number of moles of NaCl used. [1]
(b) Use your answer to calculate the concentration of the solution in mol/dm3. [2]
Answer Details
Diluting ammonia: Dilution changes concentration because the same amount of solute occupies a larger volume.
- (a) The number of moles, or amount, of ammonia transferred remains unchanged. [1]
- (b) The volume increases from 10.0 cm3 to 100 cm3, a factor of 10: \[c=2.00\times\frac{10.0}{100}=0.200\ \mathrm{mol\ dm^{-3}}\] [2]
- (c) A pipette measures the required volume more accurately or precisely than a measuring cylinder. [1]
Sodium chloride solution:
- (a) \[n=\frac{2.92}{58.5}=0.0499\ \mathrm{mol}\] This may be given as 0.050 mol. [1]
- (b) \[250\ \mathrm{cm^3}=0.250\ \mathrm{dm^3}\] \[c=\frac{0.050}{0.250}=0.200\ \mathrm{mol\ dm^{-3}}\] [2]
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