A laboratory assistant is preparing a standard solution before analysing a fertilizer extract. Fig. 1 shows solid sodium carbonate being transferred from a ...
Question 1 Report
A laboratory assistant is preparing a standard solution before analysing a fertilizer extract. Fig. 1 shows solid sodium carbonate being transferred from a weighing boat into a volumetric flask. The flask is then filled with water until the bottom of the meniscus is on the calibration line. Sodium carbonate is suitable because it is a stable solid that can neutralise sulfuric acid.
(a) Name the piece of apparatus used to measure the mass of sodium carbonate. [1]
(b) Describe how the assistant should make sure the final volume is exactly 250 cm3. [2]
(c) Complete the balanced equation for the reaction with sulfuric acid.
Na2CO3 + H2SO4 → Na2SO4 + H2O + ............ [1]
(d) Use the equation to state the amount of sulfuric acid that reacts with 1 mol of sodium carbonate. [1]
(e) Explain why the sodium carbonate must be completely dissolved before the solution is made up to the calibration line. [2]
(f) Give one safety precaution when making this solution. [2]
Answer Details
(a) Use a balance, such as an electronic balance, to measure the sodium carbonate mass. [1]
(b) Add water until the level is close to the calibration mark. Then use a dropper to add water until the bottom of the meniscus is level with the calibration line when viewed at eye level. [2]
(c) \[\mathrm{Na_2CO_3+H_2SO_4\rightarrow Na_2SO_4+H_2O+CO_2}\]
The missing product is \(\mathrm{CO_2}\). [1]
(d) The equation shows that 1 mol of sulfuric acid reacts with 1 mol of sodium carbonate. [1]
(e) Sodium carbonate must dissolve completely so it is distributed uniformly throughout the solution. If solid remains undissolved, the stated concentration is incorrect because not all of the measured sodium carbonate has been properly included in the final solution. [2]
(f) Wear eye protection and gloves. Also avoid raising sodium carbonate dust by transferring the solid carefully with a spatula. Any two valid precautions gain credit. [2]
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