Chemical quantities (AS)
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You said "Your method should involve measurements of the gas produced in the reaction and you need to calculate the amount of acid needed to be in excess." I don't how to do this. The equation would be Mg (s) + 2HCl(aq) ->MgCl2(aq)+ H2(g)....that's a 1:2 reaction, if I'm right? But where do I go from there in my calculations?
210208
Corrie writes ....
From your equation 1 mole of Mg (24 g) requires 2 mole of HCl to react completely and should produce 1 mole (24000 cm3 at room temperature and pressure) of hydrogen gas, H2(g).
Let's say you want to produce 100 cm3 of hydrogen gas, e.g. to fill a gas syringe. From the information above you can work out how much magnesium to use.
Now you need to work out how much dilute HCl to use to ensure that there is a good excess of acid (so that the acid concentration doesn't change significantly during the gas measurement). Knowing the moles of Mg you intend to use, from your first calculation above, you therefore know how many moles of HCl are required to react exactly with the Mg. Multiply this by at least 5 to ensure a good excess. Now you know how many moles of HCl you need. But HCl comes as a solution. So you need to know on the concentration and volume to use.
The volume of acid you will need (in cm3) can be obtained using the formula:
Moles HCl needed = [Vol HCl (cm3)/1000] x Conc. HCl (mol dm-3)
1 M HCl will require a smaller volume of acid, and produce a faster reaction. 0.1 M HCl will require a greater volume but will give slower reaction. You'll have to do some trial runs to see which works best, taking care to minimise any temperature change that occurs at the same time.
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updated: 21 February 2008
