[thetada]

I'm shamefully unsure of the rules dictating how to select the appropriate numbers of significant figures in calculations. It is a new feature of the UK A-level that students are expected to make such judgments, where in the past they could generally assume that any answer should be quoted to three significant figures. The point, I believe, is that there's no point in calculating with 6sf of Avogadro's constant if your measurements were only accurate to 3sf. So far so good, but in the A-level data sheet, the relative mass of hydrogen is quoted as 1.0. Does that mean that the answer to any molar calculation involving the relative mass of hydrogen should be quoted to 2sf?

[AWK]

... the relative mass of hydrogen is quoted as 1.0. Does that mean that the answer to any molar calculation involving the relative mass of hydrogen should be quoted to 2sf?

Your reasoning is correct.

[thetada]

Thanks. I'm concerned this point might be lost on some examiners.