Principles of Chemistry
8 marks · NormalThis question is about calculating relative formula masses (Mr) and converting between cm³ and dm³.
Sodium carbonate decahydrate, Na2CO3·10H2O, is used in cleaning products. Calculate the relative formula mass (Mr) of sodium carbonate decahydrate. (Ar: Na = 23, C = 12, O = 16, H = 1)
A student dissolves 5.7 g of sodium carbonate decahydrate in enough water to make 250 cm³ of solution. Calculate the concentration of the solution in g/dm³.
Convert 250 cm³ to dm³. Show your working.
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) 286 · must show All terms calculated and summed correctly
(b) 22.8 g/dm³ · must show Correct method and units
(c) 0.250 dm³ · must show Divide by 1000
Common mistakes: Forgetting to multiply quantities in brackets in (Mr); Incorrectly converting cm³ to dm³ (not dividing by 1000)
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correctly calculates the sodium carbonate part of the Mr (Na2CO3 = 106).
M2 (a) · 1 mark · Correctly calculates the water of crystallisation part (10H2O = 180).
A1 (a) · 1 mark · Correct total Mr (286).
M3 (b) · 1 mark · Calculates scale up from 250 cm³ to 1000 cm³. (follow-through)
M4 (b) · 1 mark · Correct division or multiplication shown (×4). (follow-through)
A2 (b) · 1 mark · States correct answer with units (22.8 g/dm³). (follow-through)
M5 (c) · 1 mark · Shows or states correct conversion method (÷1000).
A3 (c) · 1 mark · Gives correct value of 0.250 dm³.
Worked solution
a. Na2CO3: (2 × 23) + 12 + (3 × 16) = 46 + 12 + 48 = 106 10H2O: 10 × [(2 × 1) + 16] = 10 × 18 = 180 Total Mr = 106 + 180 = 286
b. Mass dissolved = 5.7 g in 250 cm³ To find g/dm³: 5.7 g in 250 cm³ is equivalent to (5.7 × 1000) / 250 = 22.8 g/dm³
c. To convert 250 cm³ to dm³: 250 ÷ 1000 = 0.250 dm³