Question

You have a .025g Lithium ribbon and excess HCl. Using these 2, the reaction is:
2Li(s) + 2HCl(aq) -> 2LiCl(aq) + H2(g)

You collect 45.5 mL hydrogen gas at a pressure of .969 atm (after correcting for water vapor pressure) at 25 C*. Calculate the experimental molar mass of lithium based on your gathered data.
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Please guide me step by step on this, I'd appreciate it!

Answer 1

Work backward. To get the molar mass, you need moles and mass. You're given mass of Li, so that's useful. You need moles. From where can you get moles? Well, you're given a chemical reaction, and told how much product you generate if you use up all the Li. If you can get the moles of the product, you can use the chemical equation to find the moles of Li. Alas, you're not told the moles of H2 produced, but rather the volume at a given pressure and temperature. Hmm...what connects moles of a gas with its volume, pressure and temperature? The equation of state. Assuming the H2 behaves ideally, that would be pV = nRT. So, solve that equation for n (the number of moles). Plug in p,T, and V, being very careful to use consistent units and keeping track of sig figs. Now you know moles of H2 produced. You can see from the chemical equation you need 2 moles of Li for each mole of H2 produced, so take your moles of H2 and multiply by 2 to get moles of Li consumed. (The 2 is an exact number, so sig figs stay the same.) Now you know moles of Li, and you were given the mass. Calculate molar mass, keeping track of sig figs, and round appropriately. Fetch a cool beer from the fridge and celebrate.