1. 1
   A student wants to calculate the percentage by mass of oxygen in water, H2O (Mr = 18, using Ar values H = 1 and O = 16), for a chemistry report.

   (a)

   Which formula should the student use?

   [1 mark]

   • A% = (Ar x number of atoms of that element / Mr of compound) x 100
   • B% = (Mr of compound / Ar of element) x 100
   • C% = Ar of element x Mr of compound
   • D% = number of atoms of that element / 100

   (b)

   How many oxygen atoms are present in one formula unit of water, H2O?

   [1 mark]

   • A1
   • B2
   • C3
   • D0

   (c)

   Calculate the percentage by mass of oxygen in water, H2O, giving your answer to one decimal place and showing your working.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A farmer wants to compare the nitrogen content of ammonium nitrate fertiliser, NH4NO3 (Mr = 80), using Ar values N = 14, H = 1 and O = 16.

   (a)

   How many nitrogen atoms are present in one formula unit of NH4NO3?

   [1 mark]

   • A2
   • B1
   • C3
   • D4

   (b)

   Which calculation correctly confirms that the relative formula mass of NH4NO3 is 80?

   [1 mark]

   • A(2 x 14) + (4 x 1) + (3 x 16) = 80
   • B(1 x 14) + (4 x 1) + (3 x 16) = 66
   • C14 + 1 + 16 = 31
   • D(2 x 14) + (3 x 1) + (4 x 16) = 95

   (c)

   Calculate the percentage by mass of nitrogen in ammonium nitrate, NH4NO3, showing your working.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A nutritionist is comparing two calcium supplement tablets: one made from calcium carbonate, CaCO3 (Mr = 100), and one made from calcium chloride, CaCl2 (Mr = 111). Ar values: Ca = 40, C = 12, O = 16, Cl = 35.5.

   (a)

   A nutritionist is comparing calcium supplement tablets made from calcium carbonate, CaCO3 (Mr = 100), and calcium chloride, CaCl2 (Mr = 111). Calculate the percentage by mass of calcium in calcium carbonate, CaCO3, showing your working. (Ar: Ca = 40, C = 12, O = 16, Cl = 35.5)

   [3 marks]

   (b)

   Calculate the percentage by mass of calcium in calcium chloride, CaCl2, showing your working, to one decimal place, and state which of the two supplements (calcium carbonate or calcium chloride) provides more calcium per gram of tablet.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fertiliser company is comparing the nitrogen content of two products to decide which delivers nitrogen to crops more efficiently: ammonium sulfate, (NH4)2SO4 (Mr = 132), and urea, CO(NH2)2 (Mr = 60). Ar values: N = 14, H = 1, C = 12, O = 16, S = 32.

   (a)

   Calculate the percentage by mass of nitrogen in ammonium sulfate, showing full working. (Ar: N = 14, H = 1, S = 32, O = 16)

   [6 marks]

   (b)

   Calculate the percentage by mass of nitrogen in urea, CO(NH2)2, showing full working, and use both percentage calculations to evaluate which fertiliser is likely to be more efficient in terms of the mass that needs to be transported and applied to deliver a given mass of nitrogen to crops.

   [6 marks]

   Total for question 4: 12 marks

End of questions