[ChennistryNewbie]

Is it 24g Mg?

[mikasaur]

You have to convert 5g Mg to moles Mg.

[ChennistryNewbie]

You have to convert 5g Mg to moles Mg.

is it

1mole of Mg = 24g
        ?         = 5g


5 x 1 divide 24 = 0.208

[ChennistryNewbie]

but it says I need to calculate the volume of Hydrogen gas produced at STP , when 5g of Mg react with HCl

[Borek]

1mole of Mg = 24g
        ?         = 5g


5 x 1 divide 24 = 0.208

OK, you know number of moles of Mg that reacted. Look at the balanced reaction equation - do you know how to read it? Can you use it to calculate number of moles of hydrogen than were produced?

[ChennistryNewbie]

1 mole of Mg = 1 mole of H₂
       0.208     =     ?

I don't think it makes sense :/

[Arkcon]

Please stop quoting the quote of a quote of a quote.   This is a thread, we can read the last posting.  You're making the thread longer for no reason.

Why do you think 0.208 moles makes no sense?  What if I said it was 0.500 moles?  That's a half of a mole, you could have a half of a mole of magnesium, you just said one mole of magnesium was 24 grams, you can have half a mole, and you'd know it's mass too.

That's why I told you, a balanced equation doesn't say: pick up one atom of magnesium, and smush 2 molecules of HCl into it, although new students are often told to visualize chemical reactions that way.  The units of chemical equations are always moles, because that's what we work with.

Now, you've determined moles from the mass, and now you have moles of H₂.  They wouldn't have given you this problem if you didn't also have a formula that relates moles of hydrogen gas to mL of volume.  Do you know that formula?

[ChennistryNewbie]

Ok , so Mg is 0.208

Now we need to try and find H2

Formulas that I know are the Avogado Constant ; 6.02x10₂₃
                                and volume ; 22.4dm₃

[mikasaur]

Yes, now you need to find how much H₂ is produced.

As Borek stated, you can look at the balanced chemical equation and if you know how to read it you can calculate the moles of H₂ produced by your 0.208 moles Mg.

Then as Arkcon stated you can relate the moles of hydrogen gas to mL of volume. How would you do that?

^{edit for typos 2015-10-24 11:01am PDT}

[Arkcon]

I wouldn't have used Avogadro's constant for this case, but I suppose you can derive the formula from that.  Maybe you can check your class notes or text book for an easier formula to use.

[ChennistryNewbie]

Mg (s) + 2 HCl (aq) --> MgCl 2 (aq) + H 2
0.208                                                  ?

I don't know what to do now

[Borek]

Have you read the page linked to earlier? It explains the meaning of stoichiometric coefficients.

Your approach so far is passively aggressive. You have not shown any real effort at solving the problem, instead you are whining "I don't know", ignoring hints given, and repeating the same information again and again. I am afraid it won't make you many friends, quite the opposite - people seeing your approach will soon start ignoring you.

[ChennistryNewbie]

I think the answer should be somewhere near 0.4***

[ChennistryNewbie]

I think I got it

0.208 division 5 = 0.0416

is it right?

[sjb]

I think I got it

0.208 division 5 = 0.0416

is it right?

Why divide by 5?