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
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. :-)
Correct! The mass of zinc needed to produce 115 mL of hydrogen is 0.33 g (corrected to 2 significant figures), and the number of moles of HCl needed for the reaction is 0.01 moles (also corrected to 2 significant figures).
To determine the mass of zinc needed to produce 115 mL of hydrogen, we need to follow these steps:
1. Start with the given volume of hydrogen (115 mL).
2. Convert the volume of hydrogen to moles. Since the reaction is carried out at STP (Standard Temperature and Pressure), we can use the molar volume of a gas at STP, which is approximately 22.4 L/mol.
- 115 mL / 1000 = 0.115 L
- 0.115 L / 22.4 L/mol = 0.0051 moles of H2
Next, we need to consider the stoichiometry of the equation to determine the moles of zinc required:
3. Examine the balanced equation: Zn(s) + 2HCl(l) → ZnCl2(s) + H2(g)
- The ratio between HCl and H2 is 2:1. This means for every 2 moles of HCl, 1 mole of H2 is produced.
- Since we have 0.0051 moles of H2, we need 2 times that amount of HCl.
- 2 * 0.0051 = 0.01 moles of HCl
Finally, we can determine the mass of zinc needed:
4. Examine the balanced equation again: Zn(s) + 2HCl(l) → ZnCl2(s) + H2(g)
- The ratio between zinc and hydrogen is 1:1. This means for every mole of H2, we need 1 mole of zinc.
- Since we have 0.0051 moles of H2, we need the same amount of zinc.
- 0.0051 moles of Zn * molar mass of Zn = mass of zinc needed
- The molar mass of zinc (Zn) is approximately 65 grams/mole.
- 0.0051 moles * 65 grams/mole = 0.3315 grams
Rounding to the appropriate significant figures, the mass of zinc needed to produce 115 mL of hydrogen is approximately 0.33 grams.
For the number of moles of HCl needed for the reaction, we can follow a similar approach:
1. Since we already determined that we need 0.01 moles of HCl for every 0.0051 moles of hydrogen, we can multiply the required moles of HCl by 2 to account for the stoichiometric ratio.
- 2 * 0.0051 = 0.01 moles of HCl
Rounding to the appropriate significant figures, the number of moles of HCl needed for the reaction is approximately 0.01 moles.