Number 2
4 marks · NormalA scientific magazine gives the following information:
| Object | Diameter (m) |
|---|---|
| Dust particle | 0.000 0031 |
| Red blood cell | 0.000 0072 |
| Water molecule | 0.000 000 27 |
Write the diameter of the red blood cell in standard form.
Express the diameter of the water molecule as an ordinary number.
Calculate how many dust particles, placed end to end, would fit across the diameter of a red blood cell. Give your answer to 2 significant figures.
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) 7.2 × 10^{-6}
(b) 0.000 000 27
(c) 2.3
Common mistakes: Using wrong powers of 10; Mixing up numerators/denominators
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Writes 0.000 0072 in standard form correctly as 7.2 × 10^{-6}
A2 (b) · 1 mark · Correctly writes 2.7 × 10^{-7} as 0.000 000 27
M1 (c) · 1 mark · Divides the diameter of red blood cell by dust particle (accept working in decimal or standard form)
A3 (c) · 1 mark · Gives answer to 2 significant figures: 2.3
Worked solution
a) 0.000 0072 = 7.2 × 10^{-6}
b) The ordinary number for 2.7 × 10^{-7} is 0.000 000 27
c) Number = 0.000 0072 ÷ 0.000 0031 = 2.322... ≈ 2.3 (2 s.f.)