stoich help

[star8mouse]

If someone could please explain how exactly to calculate:
-what happens to the mol and conc. when you take aliquots (e.g 20 ml from 250ml)
-how to work out the mol and conc. when solutions are diluted
-when to use equations to work stuff out (esp when the equations aren't given to you)
etc.

Honestly don't know what I'm doing at all when it comes to this topic.
Any help would be much appreciated. Thanks!

[grannysmith]

If someone could please explain how exactly to calculate:
-what happens to the mol and conc. when you take aliquots (e.g 20 ml from 250ml)
-how to work out the mol and conc. when solutions are diluted
-when to use equations to work stuff out (esp when the equations aren't given to you)
etc.

Honestly don't know what I'm doing at all when it comes to this topic.
Any help would be much appreciated. Thanks!

1. Number of moles is less, but concentration remains the same. As 'c' remains constant, you can easily work out no. moles using c=m/v.
2. You could use c₁v₁=c₂v₂. Then using c=m/v, you can easily work out no. of moles if the volume is known
3. Try extracting the data, and basically write down all the data that is given to you in the question stem e.g. v=?, c=?, m=? etc.

Are you doing units 1/2 or 3/4?

[star8mouse]

Thanks! I'm doing 3/4 at the moment

[star8mouse]

Just wondering when you see a question for stoich, is there a particular approach you take or order you go in? I often just feel like I'm staring at a question with no clue as to what to do

[grannysmith]

Not sure why I was downvoted..

Thanks! I'm doing 3/4 at the moment

No problem

Just wondering when you see a question for stoich, is there a particular approach you take or order you go in? I often just feel like I'm staring at a question with no clue as to what to do

Well, usually I'd write down a balanced chemical equation (if appropriate). Then I'd work out the number of moles of each (known) substance. Using the known amounts, you can use a mole ratio to solve for any unknown amounts of a substance. This is probably the most common and straightforward way of approaching a question.

Hope this helps

[star8mouse]

Okay, thank you so much!