# Helppppp Stoichiometry (sp?)

1. Feb 7, 2005

### Samantha

1. Aluminum chloride (15.0g) + sodium hydroxide (15g) = double displacement reaction.Predict mass of aluminum hydroxide produced. What mass of the excess reagent remains unreacted?

2. What mass of copper(II) sulfate pentahydrate is needed to prepare 150.0 mL of a 0.125mol/L solution?

How do I find them?

2. Feb 7, 2005

### MusicMonkey

2. convert 150 ml to L then multiply Volume * Concentration. Therefore = (0.150 L)(0.125 mol/L). This will get you the number of moles of copper (II) sulfate pentahydrate. Then multiply the Molecular Weight of copper (II) sulfate pentahydrate by the number of moles and you will get the number of grams. Hope that helps.

3. Feb 7, 2005

### Samantha

does the Molecular Weight of copper (II) sulfate pentahydrate = 164.21g/mol?

4. Feb 7, 2005

### MusicMonkey

Not sure, if you calculated it using the molecular weights then it's probably right.

5. Feb 7, 2005

### Samantha

copper(II) sulfate pentahydrate

This is what I did:

(molar mass of Cu * 2 ) + ( molar mass of S) + (molar mass of H * 5

6. Feb 7, 2005

### MusicMonkey

Cu+S+O*4+5*(H*2+O)

The molecular formula is CuSO4(H2O)5

7. Feb 7, 2005

### MusicMonkey

Therefore your molecular weight is incorrect. Try again.

8. Feb 7, 2005

### Samantha

I re-calculated it and it came out to 249.71g/mol??

9. Feb 7, 2005

### dextercioby

It should come out 250.

What about the first problem??Can u do it...?

Daniel.

Last edited: Feb 7, 2005
10. Feb 7, 2005

### MusicMonkey

63.546+32.066+15.9994*4+5*(1.00794*2+15.9994)
I believe it should come out to be 249.686

11. Feb 7, 2005

### Samantha

1. Is this right:

Mol of AlCl3 = 0.112mol
Mol of NaOH = 0.125mol

mass = (0.112mol)(78.03g/mol)
= 8.74g

b)do I multiply 0.125 mol by the mols of 3NaCl. Then subtract 8.74 from the anwser?

12. Feb 7, 2005

### MusicMonkey

Depending on the number of the significant figures used I believe that your answer should be accurate enough.

13. Feb 7, 2005

### MusicMonkey

The final answer for question 2 should be 4.68 g.

14. Feb 7, 2005

### dextercioby

No,no,no,no,no.No such precision required whatsoever.YOUR FINAL ANSWER IS APPROXIMABLE TO 250... :tongue2:

Daniel.

15. Feb 7, 2005

### dextercioby

The remaining mass is of NaOH indeed & is roughly 1.5g

Daniel.

16. Feb 7, 2005

### Samantha

How did you get that?

17. Feb 7, 2005

### dextercioby

1 mole AlCl_{3}----------------->133.5g
1 mole NaOH-------------------------> 40g

The chemical reaction:
$$3NaOH+AlCl_{3}\rightarrow Al(OH)_{3}+3NaCl$$

So
3 moles NaOH/120g react with 1mole AlCl_{3}/133.5 g.
x g NaOH react with 15g of AlCl_{3}

Find "x" & subtract it from 15.

Daniel.

18. Feb 7, 2005

### Samantha

19. Feb 7, 2005

### Gokul43201

Staff Emeritus
Samantha, before solving problems on stoichiometry, you need to learn how to write and balance equations. I suggest you first work on this, and get your basics clear. It may take a while but it's very important, and essential to your being able to solve any kind of chemistry problem.

20. Feb 7, 2005

### Samantha

I got the same equation but I don't get the calculations..I should work on that.