[steph_r]

Helllo budding chemists!

Im having trouble understanding this particular concept of stoichiometry, involving gases. Here is the question:

Hydrogen reacts with chlorine gas according the equation 

H₂(g) + Cl₂(g)  2HCl(g)

3.0L of Hydrogen and 7.0L of chlorine are allowed to react as much as possible. What will be the volume of the gaseous mixture at the end, assuming volumes are measured at the same temp. and pressure?

I understand that:
- same temperature and pressure means STP - 22.4L, and
- Ratios can be used in a balanced equation to calculate volumes.

Do you add 7.0L and 3.0L and then multiply by 2?

Step by step explanation would be greatly appreciated.

Cheers and thank you in advance =D

[sjb]

At a very simple level, you have 1 mol of hydrogen reacting with 1 mol of chlorine to give 2 mol of hydrogen chloride gas.

As you do not have an equilibrium constant, you cannot tell the extent of the reaction, but conversely you do not need one, as the number of moles of gas will not change over the reaction. As the pressure and temperature will not change, p, n and T do not change in the reaction, and as R is a constant, that doesn't change, so what happens to V?


pV = nRT

[Astrokel]

- Ratios can be used in a balanced equation to calculate volumes.

This is Avogadro's law and it states that under same conditions of temeprature and pressure, the volume of gases is directly proportional to their moles. (Like Sjb has said)

And they want the gaseous mixture at the end so the excess reactant have to be included.

[steph_r]

I appreciate your help and it has really clarified things, however what will the actual volume be? I was thinking 20L but the answer says 10L?

Would i be able to have some assistance with the working out?

Cheers

[Astrokel]

Sjb has given a very clear statement the relation between V and moles.

For instance, 2H₂(g) + O₂(g) ----> 2H₂O(g)

According to Avogadro's law,
2 moles of hydrogen reacts with 1 mole of oxygen to form 2 moles of water
2 dm³ of hydrogen reacts with 1 dm³ of oxygen forming 2 dm³ of water.
2 cm³ of hydrogren reacts with 1 cm³ of oxygen to form 2 cm³ of water

Based on Avogadro's law, what is the volume of HCl formed?(identify limiting reagent first) Which reactant is in excess? Sum of the volume of HCl formed and the excess will be what you are interested in.

[steph_r]

Thank u 

[sjb]

Sjb has given a very clear statement the relation between V and moles.

For instance, 2H₂(g) + O₂(g) ----> 2H₂O(g)

According to Avogadro's law,
2 moles of hydrogen reacts with 1 mole of oxygen to form 2 moles of water
2 dm³ of hydrogen reacts with 1 dm³ of oxygen forming 2 dm³ of water.
2 cm³ of hydrogren reacts with 1 cm³ of oxygen to form 2 cm³ of water

Based on Avogadro's law, what is the volume of HCl formed?(identify limiting reagent first) Which reactant is in excess? Sum of the volume of HCl formed and the excess will be what you are interested in.

Bear in mind though, that is 2 dm³ of gaseous water etc.

I'm not sure that you need to actually know the volume of HCl formed (in this case) as the number of moles does not change over the reaction 1 + 1  2, and as the question says "allowed to react as much as possible". Why not try it if 10% react, 20%, 30%, .. 100% etc