State the meaning of the term 'relative atomic mass' (Ar).
(Total for Question 1 is 1 mark)
2
Give the relative formula mass, Mr, of sodium chloride, NaCl. (Ar: Na = 23, Cl = 35.5)
(Total for Question 2 is 1 mark)
3
A molecule of water is H2O. Use the Ar values H = 1, O = 16 to calculate the Mr of water.
(Total for Question 3 is 2 marks)
4
A student has 36 g of water. Calculate how many moles of water this is. Use Mr of water = 18 and the equation moles = mass / Mr.
(Total for Question 4 is 2 marks)
5
Magnesium oxide has formula MgO. A student reacts 24 g of magnesium with excess oxygen to form magnesium oxide. Calculate the mass of magnesium oxide produced. (Ar: Mg = 24, O = 16) Note: assume all magnesium reacts and conservation of mass applies.
(Total for Question 5 is 3 marks)
6
State the meaning of 'concentration' of a solution in terms of amount of substance.
(Total for Question 6 is 2 marks)
7
A pupil dissolves 0.50 mol of sodium chloride, NaCl, in water and makes the solution up to a total volume of 0.50 dm3. Calculate the concentration in mol per dm3. (Use moles / volume.)
(Total for Question 7 is 3 marks)
8
Sulfuric acid, H2SO4, is used to make a 0.20 mol per dm3 solution. Calculate the number of moles of H2SO4 needed to make 250 cm3 of this solution. (Remember 1000 cm3 = 1 dm3.)
(Total for Question 8 is 3 marks)
9
A reaction between zinc and hydrochloric acid produces hydrogen gas. Write the symbol equation for the reaction: zinc + hydrochloric acid -> zinc chloride + hydrogen.
(Total for Question 9 is 2 marks)
10
A sample contains 0.25 mol of hydrogen gas. Calculate the number of molecules of hydrogen present. Use Avogadro constant = 6.0 x 1023 mol-1. (Give your answer in standard form.)
(Total for Question 10 is 2 marks)
11
Masses are often conserved in chemical reactions. A student reacts 2.0 g of carbon with 5.5 g of oxygen to make carbon dioxide. Calculate the mass of carbon dioxide formed.
(Total for Question 11 is 2 marks)
12
Dilution: A teacher has 1.0 mol per dm3 hydrochloric acid solution. She dilutes 40 cm3 of this solution to a total volume of 200 cm3. Calculate the new concentration in mol per dm3.
(Total for Question 12 is 3 marks)
13
A student has a 0.10 mol per dm3 solution of sodium hydroxide, NaOH. How many moles of NaOH are in 100 cm3 of this solution?
(Total for Question 13 is 2 marks)
14
A teacher asks pupils to find the mass of one mole of calcium carbonate, CaCO3. Calculate its Mr and then the mass of 2.5 mol of CaCO3. (Ar: Ca = 40, C = 12, O = 16.)
(Total for Question 14 is 4 marks)
15
A simple titration gives the following results for the volume of acid used to neutralise a fixed volume of alkali: 23.4 cm3, 23.6 cm3, 23.2 cm3. Calculate the mean volume of acid used. Give the answer to 3 significant figures.
(Total for Question 15 is 3 marks)
16
Explain how you would carry out a titration to find the concentration of a hydrochloric acid solution using a standard 0.10 mol per dm3 sodium carbonate solution. In your answer describe the key steps, the measurements you would make, and two sources of error and how to reduce them.
(Total for Question 16 is 6 marks)
Mark scheme · C3F Quantitative Chemistry: Foundation Tier Practice
Question 1
B1 the average mass of an atom of an element compared with 1/12 of an atom of carbon-12, oe
Answer: Average mass of an atom of the element compared with 1/12 of carbon-12.
Question 2
B1 Mr = 23 + 35.5 = 58.5 cao
Answer: 58.5
Question 3
M1 substitutes numbers: Mr = (2 x 1) + 16
A1 Mr = 18 cao
Answer: 18
Question 4
M1 substitutes into moles = mass / Mr: moles = 36 / 18
A1 2.0 mol cao
Answer: 2.0 mol
Question 5
M1 calculate moles of Mg: moles = 24 / 24 = 1
M1 use reaction ratio Mg : MgO = 1 : 1 so moles MgO = 1
A1 mass MgO = moles x Mr = 1 x (24 + 16) = 40 g cao
Answer: 40 g
Question 6
B1 amount of solute (in moles) dissolved per unit volume of solution, usually mol per dm3, oe
B1 can be described as moles of solute per dm3 of solution, oe
Answer: Amount of solute (in moles) dissolved per unit volume of solution, mol per dm3.
Level 3 (5-6): Detailed method with correct use of a burette and pipette, clear description of measurements and calculations, and two valid sources of error with appropriate ways to reduce them.
Level 2 (3-4): Method is mostly correct with some detail missing; includes at least one correct calculation and one source of error with a way to reduce it.
Level 1 (1-2): Only basic steps given or brief description of titration with limited or no evaluation of errors.
Indicative content:
Use a pipette to transfer a measured volume (for example 25.0 cm3) of the HCl sample to a conical flask and add a few drops of an appropriate indicator such as methyl orange or phenolphthalein.
Fill a burette with the standard 0.10 mol per dm3 sodium carbonate solution and record the initial burette reading.
Titrate by adding the carbonate from the burette to the acid while swirling the flask until the indicator shows the end point; record the final burette reading and calculate the volume used (final - initial).
Repeat the titration at least three times and use concordant results to find a mean volume; use the mean to reduce random error.
Calculate moles of Na2CO3 added using moles = concentration x volume (convert cm3 to dm3), use the mole ratio between Na2CO3 and HCl to find moles of HCl, then calculate concentration of HCl from moles / volume of acid pipetted.
Sources of error: parallax reading error at the burette (reduce by reading at eye level and using consistent readings), overshooting the end point (reduce by adding indicator near the end and adding solution dropwise), impure standard or inaccurate pipette (reduce by using calibrated glassware and standard solution)