calculating mass

"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"

any help please??

Reply 1

Neuth

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"

any help please??

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

Have a think about molar ratios and work out how many moles of Hydrogen would occupy that volume of gas :smile:

Reply 2

tomlfc

Original post by Neuth

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

Have a think about molar ratios and work out how many moles of Hydrogen would occupy that volume of gas :smile:

i don't know where to start, the wording of questions just throws me

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? :h:

Reply 5

tomlfc

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? :h:

its 2:1?

Reply 6

Neuth

Original post by tomlfc

its 2:1?

Indeed, and how many moles of Hydrogen would occupy that volume of gas?

Reply 7

tomlfc

Original post by Neuth

Indeed, and how many moles of Hydrogen would occupy that volume of gas?

Reply 8

Neuth

Original post by tomlfc

No...

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

Try again by rearranging the equation and inputting the information you know :h:

Reply 9

tomlfc

Original post by Neuth

No...

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

Try again by rearranging the equation and inputting the information you know :h:

originally i got 0.416 moles an then i didnt know where to go from there with it, id done 250/24

Reply 10

Neuth

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? :smile:

Reply 11

tomlfc

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? :smile:

something to do with the ratio being 2:1?