[ChennistryNewbie]

the 5 is the 5g of Mg , the answer should be somewhere near 0.4*** , idk whats wrong

[Arkcon]

You've twice gone backwards, and twice just restated the problem.  I recognize that you can become confused.  I've also said, and Borek: seems to have agreed, you seem to want to lengthen this thread, because you think this is a chat room.  Work on that please, and don't go backwards.

Summarizing the work so far in this thread:

You've determined 0.208 moles of magnesium are consumed.  You've learned that you produce 0.208 moles of hydrogen gas.

Please look up the gas laws in your text book, class notes or online.  There is a simple formula that relates pressure, volume, moles and temperature.  The problem has defined some of those for you, you've determined moles, and you just need to calculate volume.

[Arkcon]

Oh and hey, look at this thread here:  http://www.chemicalforums.com/index.php?topic=82822.0  Its essentially the same problem.  That poster is a little bit ahead of the game, but the math will be the same.

[ChennistryNewbie]

This isn't a chatroom , I knw that . I searched up online the gas laws and this formula came ;

 PV = nRT

[Arkcon]

Good.  Now, type it again.  This time, replace the variables, P, n, R and T, with numbers that you know.  R is a constant.  But you have to pick the correct R for the units of P,V,T that you'll be using, I didn't see those in the original problem, so you'll have to check.

[Borek]

You've learned that you produce 0.208 moles of hydrogen gas.

I am not convinced he knows that.

[ChennistryNewbie]

P - Pressure we don't have any pressure to calculate

V - This is what we need to find

n - we have 0.208moles

T - we haven't got any temperature to calculate

[Arkcon]

The original problem specified STP.  Please look up that definition.  And the answer is not a brand of motor oil, nor the name of a Grunge band.

[ChennistryNewbie]

Standard Temperature and Pressure = STP

[mikasaur]

Yes standard temperature and pressure. You have a formula that needs values for T and P (and n and R). What does knowing that you are working at standard temperature and pressure tell you about the values you can use for T and P in your formula?

[ChennistryNewbie]

How do I calculate the mass of Zn required to react with HCl , to produce 3.5dm₃ of Hydrogen at STP?

Zn + HCl = ZnCl2 + H2
 ?                      3.5dm3

I got confused which formula and how to work it out

[Borek]

Simplest way to deal with stoichiometry is through the moles. Can you convert 3.5 dm³ of H₂ to number of moles of hydrogen?

[Arkcon]

ChennistryNewbie: I hope you don't mind that I merged this new question, with your older one.  Once you solve one, you'll be able to solve the others.

[ChennistryNewbie]

Simplest way to deal with stoichiometry is through the moles. Can you convert 3.5 dm³ of H₂ to number of moles of hydrogen?

I havent learnt how to convert that ... we usually do a statement and work out with cross divide multiplication

something like ; 1 mole of Zn = 1 mole of H2

[mikasaur]

I havent learnt how to convert that ... we usually do a statement and work out with cross divide multiplication

I refuse to believe that with the sorts of questions you're asking that you haven't been given a way to convert a volume of gas to moles or vice versa.

Do yourself a favor and read through your lecture notes and textbook. I'm sure there is something there to help you. If somehow it isn't, read about the Ideal Gas Law and it should become clearer. Pay particularly close attention to the last sentence in the Ideal Gas Law section of that link.