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# calculating mass watch

1. "calculate the mass of lithium that the student could add to an excess volume of water if they with to produce 250cm3 of hydrogen gas in a gas syringe.

2Li + 2H2O → 2LiOH + H2"

2. (Original post by tomlfc)
"calculate the mass of lithium that the student could add to an excess volume of water if they with to produce 250cm3 of hydrogen gas in a gas syringe.

2Li + 2H2O → 2LiOH + H2"

How have you attempted the question so far? Show us your workings, if you have any

Have a think about molar ratios and work out how many moles of Hydrogen would occupy that volume of gas
3. (Original post by Neuth)
How have you attempted the question so far? Show us your workings, if you have any

Have a think about molar ratios and work out how many moles of Hydrogen would occupy that volume of gas
i don't know where to start, the wording of questions just throws me
4. (Original post by tomlfc)
i don't know where to start, the wording of questions just throws me
Think of it worded a different way, e.g. "What's the mass of Li needed to produce 250cm^3 of H2 gas in the following equation?"

Have you tried what I suggested?
5. (Original post by Neuth)
Think of it worded a different way, e.g. "What's the mass of Li needed to produce 250cm^3 of H2 gas in the following equation?"

Have you tried what I suggested?
its 2:1?
6. (Original post by tomlfc)
its 2:1?
Indeed, and how many moles of Hydrogen would occupy that volume of gas?
7. (Original post by Neuth)
Indeed, and how many moles of Hydrogen would occupy that volume of gas?
2?
8. (Original post by tomlfc)
2?
No...

v (in cm^3) = n x 24000.

Try again by rearranging the equation and inputting the information you know
9. (Original post by Neuth)
No...

v (in cm^3) = n x 24000.

Try again by rearranging the equation and inputting the information you know
originally i got 0.416 moles an then i didnt know where to go from there with it, id done 250/24
10. (Original post by tomlfc)
originally i got 0.416 moles an then i didnt know where to go from there with it, id done 250/24
250/24 would have been correct if the volume was 250dm^3, not cm^3.

250/24000 = 0.0104

Now you know the number of moles of Hydrogen gas produced, what else do you know?
11. (Original post by Neuth)
250/24 would have been correct if the volume was 250dm^3, not cm^3.

250/24000 = 0.0104

Now you know the number of moles of Hydrogen gas produced, what else do you know?
something to do with the ratio being 2:1?
12. (Original post by tomlfc)
something to do with the ratio being 2:1?
Yes

If you know that 0.0104 are produced, you know that there will be double this amount of Lithium in the first place.

How much would that weigh?
13. (Original post by Neuth)
Yes

If you know that 0.0104 are produced, you know that there will be double this amount of Lithium in the first place.

How much would that weigh?
do you double the moles or just the mr when finding the mass? so 0.0104 x 13.9?

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Updated: October 17, 2015
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