Solve Mass to Volume Problem: Ca + 2H2O -> Ca(OH)2 + H2

[mystix]

[SOLVED] Mass to Volume Problem

I have a mass to volume sample problem in my textbox and the answer seems to be wrong. Could someone please look this over and let me know if the answer to the sample problem is in fact wrong?
Thanks very much!

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What volume of H2(g) at STP can be produced when 10.0g of calcium is reacted with excess water?

Ca(s) + 2 H2O(l) -> Ca(OH)2(s) + H2(g)

1. Balance the equation: Ca + 2 H2O -> Ca(OH)2 + 2 H2
(Where does the 2 in 2 H2 come from? This is an unbalanced equation.)
2. Find the moles of Ca reacted: mol Ca = 0.2494mol
3. Proportionality: x/2 = 0.2494mol/1 for which x = 0.4988mol
(Again the 2 from 2 H2 is used here, which is why I think the answer is wrong.)
4. Volume of H2: 0.4988mol x 22.4L/mol = 11.2L
(Shouldn’t the answer be 5.59L?)

 

[eli64]

I agree with you, there should not be a 2 infront of H2 in the equation and the answer should be 5.59L

 

[Blueshift5]

I can confirm that the answer to this mass to volume problem is indeed incorrect. The balanced equation should be Ca + 2H2O -> Ca(OH)2 + H2, where 2 moles of H2 are produced for every 1 mole of Ca reacted. Therefore, the correct proportionality should be x/2 = 0.2494 mol/1, resulting in x = 0.4988 mol. This means that the volume of H2 produced should be 0.4988 mol x 22.4 L/mol = 11.2 L, not 5.59 L. It is important to always double check the balanced equation and proportionality when solving mass to volume problems to ensure the accuracy of the answer.