Question

Zn(s) + 2HCl(l)  ZnCl2(s) + H2(g) at STP

What mass of zinc is needed to produce 115 mL of hydrogen?

- it's given 115 mL of hydrogen, so can convert to 115/1000/22,4 = 0.0051 moles H2
- the ratio between HCl and H2 is 2 : 1, so the moles of HCl needed for this reaction is 0.01
- the ratio between Zn and H2 is 1 : 1, so the moles of Zn needed for this reaction is 0.0051
so the mass of Zn need is 0.0051*65 = 0.3315 (g)

SIG FIG ANSWER 0.33G


How many moles of HCl are needed for the reaction? (2 marks)
b) 2 x 00051 = 0.01 mols

SIG FIG ANSWER 0.01 MOLS

Answer 1

The only question I would have is => why did you choose 2-sig.figs. for expressing the final answer... Typically, the final sig.figs. are used for 'practical' laboratory considerations and determination is based upon the instrument/measuring device in the experiment having the 'lowest' degree of accuracy providing data. In other words, Significant Figures are used primarily to express results in 'reasonable' dimensional magnitudes that reflect the accuracy and precision of the experiment, and that refers to practical 'data' given or obtained by measurement and not the calculation. Of course, theoretical calculations can be expressed in any degree of accuracy, but for laboratory results more 'reasonable' values are needed.

Your calculation shows 0.0051 mole, which contains 2 sig.figs. but, the 'data' value given in your problem (assume it is experimental data) is a 'measured' amount containing 3 sig.figs. (=> 115-ml), so, for micromanagement purposes (Ha!), your answers should reflect 3-sig.figs. That is => moles of H2 = 0.00510 mole and HCl = 0.0102 mole, with the last zero in the H2 value included as a sig.fig. to reflect the accuracy of the data given in the problem. :-)