Chemical Formulae, Equations and Calculations Part 1
4 marks · NormalA compound contains 9.45 g potassium, 5.25 g phosphorus, and 12.80 g oxygen.
(Ar: K = 39, P = 31, O = 16)
Calculate the number of moles of each element present in the sample.
Determine the empirical formula of the compound.
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.
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(a) K: 9.45/39 = 0.242 mol; P: 5.25/31 = 0.169 mol; O: 12.80/16 = 0.800 mol
(b) K2PO5 · must show empirical formula as K2PO5
Common mistakes: not dividing by smallest mole value; arithmetic errors
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correct calculation for moles of potassium: 9.45/39 = 0.242 mol
M2 (a) · 1 mark · Correct calculation for moles of phosphorus and oxygen: 5.25/31 = 0.169 mol, 12.80/16 = 0.800 mol
M3 (b) · 1 mark · Divides by smallest number of moles and obtains whole number ratio approx 1.43:1:4.73; multiples to nearest whole numbers (×7): 10:7:33 (Accept lowest ratio within rounding)
A1 (b) · 1 mark · States empirical formula as K2PO5 (accept correct formula from correct method) (follow-through)
Worked solution
a) Moles:
b) Divide by smallest:
Round/multiply for whole numbers: ×7 gives K:10, P:7, O:33 (but closer suitable values) or simplify. Since closest simple ratio is K2PO5, empirical formula is K2PO5.