Find the mass calculations? (1 Viewer)

[Casmira]

Anyone know how to do these effeciently? I do this really long method that only sometimes works (cross multiplication)

for example ...

Bob burns 30g of H2, how much Nitrogen is needed to make ammonia?

those sort of questions (sorry my question is a little dodgy), where you're essentially given 1 of any product or reactent and asked to find the mass/moles of another!

 

[Dreamerish*~]

I don't get your question.

I do it by logic. I write the equation, then below the product or reactant relevant in my calculations, I write the mass/volume, and the number of moles present.

 

[like_ma_style]

1. Write the equation: 3H2 + N2 --> 2NH3

2. Find the number of moles of hydrogen (n = m/M)

3. Find the number of moles of nitrogen (molar ratio is 3:1)

4. Then find the mass of nitrogen (n=m/M --> m=nM)

I think that is right :S

 

[Casmira]

Is that it?!?! Shit all these exams I lose 3 marks because I don't bother attempting them

I did a similar question for finding the mass of sulfate in a fertilizer experiment, my moles was like 0.007177366, then I used that agaisnt the molar mass of sulfate, long story cut short, the book had a different answer by a few percent because it rounded off early (used 0.007 moles), now should I round off or maintain that answer when calculating?

 

[Casmira]

anyone!? I dont want to find out that If I dont round off early it cocks up my answers

 

[like_ma_style]

Hmmm well it would be more accurate if you didn't round off. They wouldn't be cruel to take marks off because you are more accurate.

I do not round, and I don't see why you should.

Sorry I couldn't be of much more help.

 

[thunderdax]

They could mark you down for rounding off early in your answer, but never for not rounding off and using the same number in the calculator.

These questions are really pretty easy and the procedure like_ma_style used works every time.