First, I am assuming that you used up all of the magnesium in this reaction (that you had enough HCl to react with all of the magnesium).

If so, here is how to figure this out.

You start out with your mass of Mg. You divide that by the molar mass of Mg which is 23.4 g/mol. That tells you how many moles of Mg you have.

Then you have to consider that one mole of hydrogen is produced by the reaction of one mole of Mg.

So now because you know how many moles of Mg you have, you know how many moles of H. This will be (1.78x10^-2/24.3)*2.

Remember the universal gas law: PV = nRT.

That means that

V = (nRT)/P

n is your number of moles of H. R is the universal gas constant 82.1 (ml*atm)/(mol*K). T is the temperature in Kelvins. P is the pressure in atmospheres, which is 1.

Do that math and you'll have the volume in milliliters.

Once you get that, divide that by the number of seconds.

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