Calculating moles in a reaction

[EmilyB]

Looking at a question (and no, I'm not in school) I'm not too sure of the answer. This area clearly is not my forte!

4.32g of PbBr2 are produced, how many grams of Pb(||) were required to react..?

 

I worked out that there were 0.0117mols in 4.32g but the proportion of Pb was 0.006...mols. The answer they were looking for was 0.0117. Surely they made a mistake? Or am I missing something?

[hypervalent_iodine]

It would probably help you to write the reaction out to work out the ratio of Pb used to PbBr2 produced. In this case, the reaction is simply:

 

Pb + Br2 --> PbBr2

 

The coefficients out the front of each component is where we get what are called the stoicheometric ratios from. In this reaction, one molecule of Pb is required to make one molecule of PbBr2. So, if you know that you have made 0.0117 moles of PbBr2, then you must have used 0.0117 moles of lead (assuming 100% conversion).

[pavelcherepan]

You haven't shown what reaction is used to synthesize PbBr₂. I would assume, that it's the most common one:

 

2KBr(aq.)+Pb(NO₃)₂ = PbBr₂(s) + 2KNO₃(aq.)

 

4.32g of lead(II) bromide is 0.0117 mols as you have calculated, then 0.0117 mols of lead(II) nitrate is required for the reaction. Lead nitrate is created from metallic lead via the following reaction:

 

Pb + 2HNO3 = Pb(NO₃)₂ + H₂(g)

 

Since we have 0.0117 mols of lead nitrate, the same amount of metallic lead is required, 0.0117 mols or 2.48 grams.

[Sensei]

Looking at a question (and no, I'm not in school) I'm not too sure of the answer. This area clearly is not my forte!

4.32g of PbBr2 are produced, how many grams of Pb(||) were required to react..?

If they asked for grams, like you said above...

 

The answer they were looking for was 0.0117. Surely they made a mistake? Or am I missing something?

 

...they cannot accept answer in moles..

 

Units don't match.

 

I worked out that there were 0.0117mols in 4.32g

Right.

 

but the proportion of Pb was 0.006...mols.

Proportions, any kind, wouldn't be given with units..

 

to get 0.006 mol you multiplied 0.01177 mol * 0.5646 = 0.006645 mol

 

But you should multiply initial mass:

4.32 g * 0.5646 = 2.4389 g

or from moles:

0.01177 mol * 207.2 g/mol (Pb mass) = 2.4389 g

 

Ratio 0.5646+0.4354=1 is of course from mass of Pb / total mass, and mass of Br/total mass. Unitless.

 

Edited October 18, 2015 by Sensei

[studiot]

Very good (teaching) points Sensei.

 

+1

[EmilyB]

Thank you everyone! Here is the full original question

 

(apologies if it makes a difference to anything :|)