Fig. 1 shows a water company testing sodium hypochlorite bleach solution. A 5.00 cm3 sample is diluted to 250 cm3 in a volumetric flask. The diluted solutio...
Question 1 Report
Fig. 1 shows a water company testing sodium hypochlorite bleach solution. A 5.00 cm3 sample is diluted to 250 cm3 in a volumetric flask. The diluted solution has concentration 0.0120 mol/dm3. The company needs the concentration of the original bleach solution.
(a) Complete the statement: the number of moles of sodium hypochlorite is the same before and after dilution. [1]
(b) Use the dilution information to calculate the concentration of the original solution. [3]
(c) Give one reason why the original bleach should not be pipetted by mouth. [1]
(d) Name the apparatus used to make the diluted solution to an accurate final volume. [1]
(e) Suggest why the flask is stoppered before it is inverted. [1]
Answer Details
Dilution: Adding water changes the volume and concentration, but does not change the amount of sodium hypochlorite transferred into the flask.
- (a) The quantity that remains the same is the number of moles, or amount, of sodium hypochlorite. [1]
- (b) Moles in the diluted solution: \[n=cV=0.0120\times0.250=0.00300\ \mathrm{mol}\] These are the same moles originally present in 5.00 cm3: \[5.00\ \mathrm{cm^3}=0.00500\ \mathrm{dm^3}\] \[c=\frac{0.00300}{0.00500}=0.600\ \mathrm{mol\ dm^{-3}}\] [3]
- (c) Bleach must not be pipetted by mouth because it is harmful, corrosive, or toxic. [1]
- (d) Use a volumetric flask to make the final volume accurate. [1]
- (e) The flask is stoppered before inversion to prevent solution escaping or spilling. [1]
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