The combustion of elements in oxygen
4 marks · NormalThe diagram below shows a scientist burning a piece of magnesium in a bell jar filled with air. The bell jar is placed over a trough of water. After ignition, the magnesium burns in oxygen to form magnesium oxide.
| Water level | Volume / cm³ |
|---|---|
| Initial | 695 |
| Final | 830 |
After burning, the white magnesium oxide forms and dissolves in the water. The levels of water inside the bell jar before and after combustion are shown in the table.
State one observation you would see when magnesium reacts with oxygen in this experiment.
Calculate the volume of oxygen that was used up during the reaction. Show your working.
Given that the total volume of air in the bell jar at the start was 695 cm³, calculate the percentage of oxygen in the air based on your answer to (b). Show your answer to two significant figures.
Explain why there might still be some magnesium left after the experiment.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) A bright white light is seen, and a white solid forms. · must show Bright white flame, White solid (magnesium oxide) forms
(b) 830 - 695 = 135 cm³ · must show Working shown: 830 minus 695
(c) \frac{135}{695} \times 100 = 19\% · must show Correct calculation method, Answer rounded to two significant figures
(d) Magnesium is added in excess so that all the oxygen reacts; this ensures the reaction uses all available oxygen. · must show Explanation includes excess magnesium, Relates to ensuring all oxygen is used
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States a correct observation: bright white flame, or white solid forms.
M1 (b) · 1 mark · Correctly calculates volume of oxygen used (final volume minus initial volume).
A1 (c) · 1 mark · Correctly calculates the percentage of oxygen in air, using the answer to (b) and shows correct rounding. (follow-through)
B1 (d) · 1 mark · Explains excess magnesium ensures all oxygen reacts (accept wording: used up all oxygen or to prevent incomplete reaction).
Worked solution
a) When magnesium reacts with oxygen, it burns with a bright white flame and produces a white solid (magnesium oxide).
b) Volume of oxygen used = final volume - initial volume:
c) Percentage of oxygen in air = (volume of oxygen used / initial air volume) × 100
d) Some magnesium is left after the experiment because an excess is used. This ensures that all the oxygen in the bell jar is able to react, so the experiment measures the oxygen content of air accurately.