i need help at chemistry

Original post by joannacole

can someone please tell me answers/method of how you would work these out? much appreciated
1. How many cm³ of concentrated H₂SO₄ (19 M) are required to prepare 2½ dm³ 0.05 M H₂SO₄?

2. 2 dm³ of 2 M copper(II)nitrate (Cu(NO₃)₂) solution contains:1. 4 mol dm^-3 Cu²⁺
2. 4 mol NO₃^-
3. 12 mol of ions

well... on study help, we don't just do people's questions for them. We like to help them get to the answers themselves.

All these questions are about the concept of the mole. A mole of something (a compound, an element, an ion etc) is just Avogadro's number of them.

A mole of something will have a mass (in grams) which is the same as its formula weight (or m_r

So a mole of sodium atoms is 6.02 x 10²³ sodium atoms and will have a mass of 23 grams.

So, have a go at the questions and we can guide you from there.

Reply 2

OP

thanks, but sorry that makes 0 sense to me
im a beginner at all of this

Reply 3

Original post by joannacole

thanks, but sorry that makes 0 sense to me
im a beginner at all of this

Have you not been taught about moles yet? I am presuming you have if they've given these questions.

I'll give you an example.

A solution of 1M sodium chloride solution contains 1 mole of sodium chloride per dm³. We say it is a "1 molar solution" other ways of saying this are 1M (note the capital M), or 1 mol dm^-3. They all mean the same thing.

SInce sodium chloride consists of Na⁺ ions and Cl^- ions, a 1 molar solution will contain

1 mole of Na⁺ ions per dm³
and
1 mole of Cl^- ions per dm³

so in total, it contains
2 moles of ions per dm³

with me so far?

Reply 4

OP

ok, I'm with you now
thank you for your patience

Reply 5

Original post by joannacole

ok, I'm with you now
thank you for your patience

ok, so now let's look at your second question (as it's the easier one).

2 dm3 of 2 M copper(II)nitrate (Cu(NO3)2) solution contains

So we have 2M copper nitrate solution. That must contain 2 moles of copper nitrate per dm³. But we have 2 dm³, so how many moles of copper nitrate must we have?

Reply 6

OP

4?

Reply 7

Original post by joannacole

4?

good.

Now, we know that each copper nitrate contains 2 nitrate ions (you can see that from its formula), so that must mean we would have 8 moles of NO₃^- ions.

So the answer can't be number 2

Reply 8

OP

so, is it 1, 4 mol dm3 cu2+

Reply 9

Original post by joannacole

so, is it 1, 4 mol dm3 cu2+

no. If you read it carefully, it says 4 mol dm^-3 Cu²⁺ (not 4 mol Cu²⁺)

We already know it's 2 mol dm^-3 copper nitrate (because it's a 2M solution). Each copper nitrate contains only 1 Cu²⁺, so it must also be 2 mol dm^-3 Cu²⁺

So you have the answer by elimination. See if you can work out why it's the correct answer

(edited 10 years ago)

Reply 10

OP

ahh ok! you've made it very clear now, thank you! I have a chemistry module at university, and my lecturer assumes everyone did chemistry at A level, but i didn't so it's hard to understand. But thank you for all of your help :smile:

Reply 11