# Need help with stoichiometry

#### Hcanfield

Hello, I'm new here and I am taking General chemistry as my last prereq. I'd like help in understanding the steps to this problem:

How many grams of Li3N can be formed from 1.75mol of Li? Assume an excess of nitrogen.
6Li + N2 --> 2Li3N

#### studiot

So have you worked out (or looked up) the molecular masses of lithium and lithium nitride?

What do you make them to be ?

• topsquark

#### Hcanfield

So have you worked out (or looked up) the molecular masses of lithium and lithium nitride?

What do you make them to be ?
Lithium= 6.94 g/mol
Lithium nitride= 34.83 g/mol

#### studiot

Yes that's good.
Since the question gives the available mass of lithium to 2 decimal places (dp) you should work to 2 dp.
Do you understand this (it is an important side issue) ?

So your correct equation says (in words)

6 times 6.94 grams of Li makes 2 times 34.83 grams of hydride.

That is

6 x 6.94 grams makes 2 x 34.83 grams

That is

41.64 g makes 69.66 g

That is

1 g makes $$\displaystyle \frac{{41.64}}{{69.66}}$$ g

Can you finish the last line?

1.75 g makes $$\displaystyle \frac{{41.64}}{{69.66}}\;x\;1.75$$ g

#### studiot

Yes that's good.
Since the question gives the available mass of lithium to 2 decimal places (dp) you should work to 2 dp.
Do you understand this (it is an important side issue) ?

Whoopsy - I just noticed you are asked to calculate for 1.75 moles of lithium, not 1.75 grams so lets do that instead.

So your correct equation says (in words)

6 times 6.94 grams of Li makes 2 times 34.83 grams of hydride.

That is

6 x 6.94 grams makes 2 x 34.83 grams

That is

6 gram-moles makes 69.66 g

That is

1 gram-mole makes $$\displaystyle \frac{{69.66}}{6}$$ g

1.75 gram-moles makes 1.75 x $$\displaystyle \frac{{69.66}}{6}$$ g

Last edited:

#### studiot

Yes that's right.
I'm glad to see you have stated them to 2 decimal places (2dp). The question states the amount of lithium to 2 dp so this is the right thing to do.

OK so your correct equation says

6 gram moles of lithium make 2 gram moles of nitride.

6 gram moles of lithium make 2 x 34.83 grams of nitride.

6 gram moles of lithium make 69.66 grams of nitride.

1 gram mole of lithium makes $$\displaystyle \frac{{69.66}}{6}$$ grams of nitride

1.75 gram moles of lithium makes 1.75 x $$\displaystyle \frac{{69.66}}{6}$$ grams of nitride

• WMDhamnekar
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