6 exam-style questions with full mark schemes and model answers. Write your own answer and the AI examiner marks it against the mark scheme.
Learn this properly: Conservation of MassA chemical company manufactures a product using a reaction that does not go to completion and produces some unwanted by-products.
The company wants to make its process as efficient and sustainable as possible.
Explain the meaning of percentage yield and atom economy, and discuss why a company would want both to be as high as possible. (6 marks)
Calcium carbonate decomposes when heated:
CaCO3→CaO+CO2
A student heated 25.0 g of calcium carbonate until it had completely decomposed.
(Relative atomic masses: C = 12; O = 16; Ca = 40.)
(a) Calculate the number of moles of calcium carbonate in 25.0 g. (2 marks)
(b) Calculate the mass of calcium oxide (CaO) produced. Show your working. (2 marks)
Magnesium reacts with hydrochloric acid to produce hydrogen gas:
Mg+2HCl→MgCl2+H2
In an experiment, 0.050 mol of hydrogen gas was produced.
(At room temperature and pressure (rtp), 1 mole of any gas occupies 24 dm³.)
(a) Calculate the volume of hydrogen gas produced at rtp, in dm³. (2 marks)
(b) State the number of moles of magnesium that reacted to produce this hydrogen. (1 mark)
A student carried out a titration to find the concentration of a solution of sodium hydroxide (NaOH). They titrated 25.0 cm³ of the NaOH solution against hydrochloric acid of concentration 0.200 mol/dm³.
The balanced equation is: NaOH+HCl→NaCl+H2O
The student's results are shown in the table.
| Titration | Titre / cm³ |
|---|---|
| Rough | 24.60 |
| 1 | 23.40 |
| 2 | 23.50 |
| 3 | 23.45 |
(a) Using only the concordant results, calculate the mean titre of hydrochloric acid. (1 mark)
(b) Using your mean titre, calculate the concentration of the sodium hydroxide solution in mol/dm³. Show your working. (2 marks)
In an industrial reaction, the maximum (theoretical) mass of product that could be made was 80.0 g. The actual mass of product obtained was 62.0 g.
Calculate the percentage yield of the reaction. (2 marks)
A reaction is carried out in a sealed container so that no substances can enter or leave.
State the law of conservation of mass. (1 mark)