# STOICHIOMETRY- whoever gets it plz help-urgent!!!!gag (1 Viewer)

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#### n a + a l i e

##### Guest
hello, any1 understand stiochiometry..ok i memroised the formulas but im finding it so hard to use them
n i dun get excesss questions.

N

#### n a + a l i e

##### Guest
with the formulas is it reli rigid like say if u wanted to work out the mass ud use a certain formula, coz mi teacher was sayin for the formula m=n/M u don use tht to calculate volume. first q for starters but im sure ill come up with heaps mor lol, thnx

#### Dreamerish*~

n a + a l i e said:
with the formulas is it reli rigid like say if u wanted to work out the mass ud use a certain formula, coz mi teacher was sayin for the formula m=n/M u don use tht to calculate volume. first q for starters but im sure ill come up with heaps mor lol, thnx
Could you please spell properly? I don't mean to be a dickhead but it would be easier to read.

I think I get what you mean. No, you don't always have to write out the formulae when doing calculations. About half of the time I use mathematical logic in calculation questions. For example, I never feel like using the C1V1 = C2V2 rule, so I just do it logically in my head.

Of course, there is no other way of calculating moles. It's always n = m/M.

What do you mean by calculating volume?

N

#### n a + a l i e

##### Guest
isnt M , molar mass, the volume?

#### Dreamerish*~

n a + a l i e said:
isnt M , molar mass, the volume?
M = molar mass = the mass of one mole of the element or compound (in grams).

For example, the molar mass of carbon is 12 g, and the molar mass of carbon dioxide is 12 + 16 + 16 = 44 g.

#### tristan88

##### Member
Molar Mass is the relative atomic mass of the compound/element. It can be derived from the periodic table.

N

#### n a + a l i e

##### Guest
how do u manage only using the m=n/M fomula

#### Dreamerish*~

n a + a l i e said:
how do u manage only using the m=n/M fomula
If you want to find out how many moles there are in a sample of an element/compound: n = m/M

If you want to find out what the mass would be if you had a known number of moles of a certain element/compound: m = nM

If you want to find the molar mass of a compound/element of a sample with known mass and number of moles (I highly doubt you'll ever be asked this one, unless they want you to identify the element/compound): M = m/n

N

#### n a + a l i e

##### Guest
ok take 4 example..
Q. a hydrocarbon contains 82.7% carbon and 17.3% hydrogen by weight- work out its empirical formula

#### Dreamerish*~

n a + a l i e said:
ok take 4 example..
Q. a hydrocarbon contains 82.7% carbon and 17.3% hydrogen by weight- work out its empirical formula
I've completely forgotten, but I'll take a stab.

M of carbon = 12 and M of hydrogen = 1.008.

You make it into a ratio:
82.7/12 : 17.3/1.008
= 6.8917 : 17.1627 - divide both sides by 6.8917
= 1 : 2.49
= 2 : 5

C2H5?

That's a little wrong.

#### Dreamerish*~

n a + a l i e said:
hmm i think ur actually on the right track...so far im fine with th eqs that revolve around simply using the m=n/M formula, but im having difficulty with recognising and identifying the figures they give in the exams this question highlights it well, well i think

0.22 g of a gas is found to occupy .112 L at STP. find the molar mass of the gas
For this you need the following rules:

At 0°C and 1.00 atm pressure, one mole of any gas occupies 22.41 L.
At 25°C and 1.00 atm pressure, one mole of any gas occupies 24.79 L.

STP: 1 atm and 0°C.

V = m/M x 22.41
0.112 L = 0.22/M x 22.41
M = 44.

am i right

#### Dreamerish*~

xxxx8888 said:

am i right
Wrong.

Pb, not PB. Yes, there is a difference and you will be marked wrong.

N

#### n a + a l i e

##### Guest
soz. i don't really quite understand how you have manipulated the formula. so where is the n in there?

#### Dreamerish*~

n a + a l i e said:
soz. i don't really quite understand how you have manipulated the formula. so where is the n in there?
Dreamerish*~ said:
V = m/M x 22.41
0.112 L = 0.22/M x 22.41
M = 44.
n = m/M, remember?

N

#### n a + a l i e

##### Guest
but isnt teh v supposed to represent volume, n represents no of moles?

N

#### n a + a l i e

##### Guest
o wait is tht another thing, volume= m/M x conditions

#### Dreamerish*~

n a + a l i e said:
o wait is tht another thing, volume= m/M x conditions
n = m/M

V = n x 24.79 (or 22.41 - depending on conditions)

There isn't one "skeleton-key" formula. You need to memorise quite a few.

N

#### n a + a l i e

##### Guest
ook, wud u kno of any other minor formulas tht are applicable