Reaction prediction

H2SO4(aq) + Mg(s) MgSO4(aq) + H2(g)

if this reaction takes place, how would you calculate the amount of acid to be used so that the h2 gas generated will all fit into a 100 ml syringe?

does it have something to do with the fact that one mole of gas occupies 24dm-3 at r.t.p?

thanks

Reply 1

marcusfox

Calculate how many moles of gas occupy 100ml and then use the same number of moles of acid.

Marcus

Reply 2

EierVonSatan

yeah just work backwards - find how many moles would be in 100ml, which will be the same amount of acid and Mg you need, then just convert them to masses/volumes

Reply 3

sonic23

thanks!

but doesnt this essentially mean that one mole of acid will produce one mole of hydrogen gas, because i am essentially trying to prove that the acid is dibasic, and so 1 mole of the acid would produce 2 moles of the hydrogen gas.

Reply 4

EierVonSatan

this reaction gives you a 1 to 1 ratio because hydrogen exists in a dimer, if you want a 2 to 1 ratio use a monoprotic acid like HCl

Reply 5

sonic23

sorry. that just confused me?

i need to prove that sulphuric acid is disbasic

Reply 6

marcusfox

sonic23

No, because hydrogen gas is H2 and one mole of the acid H2SO4 produces one mole of gas.

If you use HCl so your reaction is 2HCl + Mg > MgCl2 + H2, you need two moles of acid to make one mole of gas.

Marcus

Reply 7

marcusfox

sonic23

sorry. that just confused me?

i need to prove that sulphuric acid is disbasic

Do you have to do it by measuring the gas? - the way I would do this is by titration.

-Do a neutralisation titration (using methyl orange or an equivalent as an indicator) of an equal amount (eg: 25ml) of 1M HNO3; 1M HCl and 1M H2SO4 (separately of course), using 1M NaOH.

-Record the volumes of NaOH needed to neutralise each.

-As they are strong acids the monoprotic acids (hydrochloric and nitric) should be within a reasonable margin for error (5%-ish), but roughly the same, showing that they have an equal proportion of H+ ions to be neutralised.

H+ + OH- > H2O

-However the diprotic sulphuric acid should react with twice as much NaOH, showing it has twice as many H+ ions.

2H+ + 2OH- > 2H2O
(^needs twice as many OH ions see)

Marcus

Reply 8

sonic23

Tahnks but i also need to do a titration.

for the volume of 1M acid to generate 100 ml gas wud tht be roughly 240 dm3
i.e 240 000 ml?

Reply 9

marcusfox

sonic23

Tahnks but i also need to do a titration.

for the volume of 1M acid to generate 100 ml gas wud tht be roughly 240 dm3
i.e 240 000 ml?

LOL - what are you like! 240 litres?

One mole of H2 occupies 22.4 dm3 or 22.4 litres. Or 22400 ml. So 100ml of gas is going to be 0.004 of a mole.

It might be fairly difficult measuring something like 0.004 moles of magnesium, but not 0.004 moles of MgCO3 according to the equation MgCO3 + H2SO4 > MgSO4 + H2O + CO2

LOL again.

Marcus

Reply 10

sonic23

lol never thought about tht ridiculous figure.

but using 0.004 moles and 1m concentration , and conc.= n of moles / v

i get 4ml of acid to produce 100 ml of gas.

tht cant be ryt neither can it?

Reply 11

marcusfox

sonic23

lol never thought about tht ridiculous figure.

but using 0.004 moles and 1m concentration , and conc.= n of moles / v

i get 4ml of acid to produce 100 ml of gas.

tht cant be ryt neither can it?

No, you use excess acid. Since it will be very difficult for all of the acid to be in contact with your reagent. Once all the reagent is used up, the reaction will stop.

Marcus

Reply 12

sonic23

oh. but im using excess magneisum to ensure all the ACID reacts since the acid is the reagent in question here ; to prove that it is dibasic.

so if i use both excess magnesium and acid, the syringe would overflow