Pharmacy calculations rounding — whole number vs decimal?
Hi, I’m revising for a pharmaceutical calculations exam and I’m trying to work out what rounding convention is expected when the question does not specify decimal places or significant figures.
I understand the maths, but I’m unsure whether final answers should be left as exact calculated values, rounded to a practical whole unit, or rounded to 1 d.p./3 s.f.
Here are examples from my practice questions.
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Example 1 — moles
How many moles of solute are there in 36 mL of a 0.85 mol/L solution?
36 mL = 0.036 L
0.85 × 0.036 = 0.0306 mol
Would you write the final answer as:
0.0306 mol or 0.031 mol?
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Example 2 — sodium ion content
How many mg of sodium ions are contained in a 1 g tablet of sodium chloride?
RMM NaCl = 23 + 35.5 = 58.5
1000 mg × 23 ÷ 58.5 = 393.162 mg Na⁺
Would you write:
393 mg or 393.2 mg?
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Example 3 — mmol of sodium
How many mmol of sodium are contained in 300 mL of 0.45% w/v NaCl?
0.45% w/v = 0.45 g/100 mL
In 300 mL:
0.45 × 300 ÷ 100 = 1.35 g NaCl
1.35 ÷ 58.5 × 1000 = 23.0769 mmol Na⁺
Would you write:
23 mmol or 23.1 mmol?
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Example 4 — potassium ion content
How many mg of potassium ions are contained in a 1.2 g tablet of potassium chloride?
RMM KCl = 39 + 35.5 = 74.5
1200 mg × 39 ÷ 74.5 = 628.1879 mg K⁺
Would you write:
628 mg or 628.2 mg?
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Example 5 — mmol of chloride
How many mmol of chloride are contained in 150 mL of 1.2% w/v NaCl?
1.2% w/v = 1.2 g/100 mL
In 150 mL:
1.2 × 150 ÷ 100 = 1.8 g NaCl
1.8 ÷ 58.5 × 1000 = 30.7692 mmol Cl⁻
Would you write:
30.8 mmol, or round to a whole mmol as 31 mmol?
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Example 6 — mmol of potassium
How many mmol of potassium are contained in 150 mL of 1.2% w/v KCl?
1.2% w/v = 1.2 g/100 mL
In 150 mL:
1.2 × 150 ÷ 100 = 1.8 g KCl
1.8 ÷ 74.5 × 1000 = 24.1611 mmol K⁺
Would you write:
24 mmol or 24.2 mmol?
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The practice papers I’m using seem inconsistent: some answers keep decimals, e.g. 12.198 g or 235.9 mg, while some answers seem to use whole mmol values.