[MakoEyes]

Here's a question I don't know the technique I would use to solve it. Can someone offer me some solution so I can observe the techniques used?

The elements nitrogen and magnesium react to give magnesium nitride according to the balanced
equation:
N2 + 3 Mg ->  Mg3N2
Suppose you have 0.4530 mol N2.
(a) How many moles of Mg are needed for complete reaction?
(b) What mass of Mg3N2, in grams, can be produced? (The molar mass of Mg3N2 is 100.9284 g/mol).

[Borek]

Do you know how to read a reaction equation?

http://www.chembuddy.com/?left=balancing-stoichiometry&right=stoichiometric-calculations

[MakoEyes]

Yes I do.

For (a), is this the technique:

0.4530 mol N2 * (3 mole Mg / 1 mole N2) = answer

? Or am I off base a bit.

[Ak]

yeah thts right and then for (b) you take either the moles of N2 or Mg (id go with N2 cuz its 1:1).  Then u have g/mol and mol and u need to get grams (g).

[MakoEyes]

yeah thts right and then for (b) you take either the moles of N2 or Mg (id go with N2 cuz its 1:1).  Then u have g/mol and mol and u need to get grams (g).

I'm a bit confused by your reply.

Here is what I did for (b)...

0.4530 mole N2 * (14.01g / 1 mole N) * (1 mole N2 / 1 mole Mg3N2) * (100.9284g / 1 mole Mg3N2) = 640.5g Mg3N2

Is this correct or did I do this wrong?

[Borek]

It is wrong. But dimensional analysis is not my forte.

[cliverlong]

Here is what I did for (b)...

0.4530 mole N2 * (14.01g / 1 mole N) * (1 mole N2 / 1 mole Mg3N2) * (100.9284g / 1 mole Mg3N2) = 640.5g Mg3N2

Is this correct or did I do this wrong?

Your lack of use of subscripts hurts my eyes - but that may be of no concern to you. More importantly, that, the arrangement of your units and not splitting the calculation into steps, makes it difficult to see where the problem is in your answer.

Rearranging your information I think reveals where the problem may lie. Your calculation:

0.4530 N₂ [mole] * (14.01 /1 [g / mole] ) * (1 N₂ / 1  Mg₃N₂ [mole / mole]) * (100.9284/ 1  Mg₃N₂ [g/mole]) = 640.5g Mg₃N₂

Looking at your left-hand side, the units work out as

[mole] * [g / mole]  * [mole / mole] *  [g/mole] =  [g²/mole]. There is a problem there.

I would look back at the single, long string of multiplications and split them into a series of steps, checking the units as I went

Clive

[MakoEyes]

Thanks Clive.. I still can't figure it out tho. I can't seem to "develop" the dimensional analysis needed to solve this problem.

Could someone show me the steps? (for b)

[cliverlong]

N₂ + 3 Mg ->  Mg₃N₂
Suppose you have 0.4530 mol N2.
(a) How many moles of Mg are needed for complete reaction?

Well,

From the equation you can see every 1 mole of N₂ will react with 3 moles of Mg.

So how many moles of Mg will react with 0.4530 mol N₂?

(b) What mass of Mg₃N₂, in grams, can be produced? (The molar mass of Mg₃N₂ is 100.9284 g/mol).

Moles again.

From the equation you can see every 1 mole of N₂ will produce 1 mole of  Mg₃N₂

So how many moles of  Mg₃N₂ will be prdoced from 0.4530 mol N₂?

What is the equation that will convert moles into mass? (you are given molar mass of Mg₃N₂)

[MakoEyes]

Thus:


0.4530 mol N2 * (3 Mole Mg / 1 Mole N2) * (100.9284 g/mol Mg2N3 / 1 mole Mg2N3) = answer?