E29 (d) Significant Figures

[Tia Tomescu 2D]

For Question E29, the question only gives a number with 3 significant figures (8.61), yet the answer for (d) on the textbook is 4 sig figs (0.3099). Does anyone know why?

[Thuy-Anh Bui 1I]

In this question, the value 8.61 g is actually not relevant. Part (d) asks "What fraction of the total mass of the sample was due to oxygen?" All the values you need for this are derived from the periodic table, not the 8.61 at the beginning of the question. To find the fraction of the mass due to O in CuCl2 * 4H2O, you would set up a fraction in this way:
[amount of O] / [molar mass of CuCl2 * 4H2O]
4(16.00 g/mol) / 206.35 g/mol = 0.3099

The numbers 16.00 g/mol and 206.35 g/mol are taken from the periodic table, and 16.00 with its 4 sig figs governs this calculation. Hope this clarifies your question.

[snehabhargava]

This is a mistake in the solution manual. You should use the sig. figs. that are given in the question and not from the periodic table because the periodic table can vary.

[Rana YT 2L]

The number of sig figs only depends on the original number of sig figs in the original values given. So you simply have to look back at the original values, but don't forget to take into consideration the fact that the rules for addition and subtraction can affect that value.