Chemistry: Solutions

[DustyMcAwesome]

Hey everyone, just a small homework assignment, hoping you could lead me in the right direction. :]

 

1. how would you make 3.00L of a 0.125M solution of Ca(NO3)2?

 

2. How many grams are in 200 mL of a 1.50 M solution Na2SO4?

 

3. How much 1.25% KBr solution can be made with 10.0 grams of KBr?

 

4. How would you make 500 grams of a 0.90% solution of NaCl from a 3.5% stock solution?

 

5. How would you make 1.00L of a 0.30M solution of KOH from a stock solution of 6.00M?

 

6. Find the new molarity if 500mL of H20 are added to 200mL of a 2.5M solution of K3PO4?

 

 

M= Molarity

 

 

 

 

I just need a step in the right direction, like which equation to use. :]

 

Thanks in advance!

[mooeypoo]

Do you remember what "M" means in units? That will help you. You should have it, for sure, in your book.

 

If you still don't know how to solve these after you see what M equals to in units, post that here, and I'll give you another hint...

[DustyMcAwesome]

M is molarity, correct?

 

We don't use a book, its all oral. I don't remember the forumlas.

 

I think one was m1v1=m2v2...........

[mooeypoo]

M is molarity, correct?

 

We don't use a book, its all oral. I don't remember the forumlas.

 

I think one was m1v1=m2v2...........

Don't think, check . Look up molarity... that's the only thing you need to solve your questions.

[DustyMcAwesome]

Wait, are you saying to look up the formulas, or the definition of molarity?

[mooeypoo]

I want the units of molarity. You can see the formula for molarity from its units. So either find me the formula of how to calculate molarity, or find me the units of molarity... either way, that's going to get you towards the solution.

 

They're easy to find on a google search try.

[DustyMcAwesome]

Molarity=mol/liter...

 

So the equation would be the molarity of the concentrated times the volume of the concentrated is equal to the molarity of the dilute times the volume of the dilute, m1v1=m2v2.

[Physicsfan]

even after you look up Molarity to your satisfaction:-) , you need to know the gram molecular mass of each compound in your questions. you must be knowing that the mass of one mole of a substance is equal to its molecular weight expressed in grams.

 

so if you take k moles of a substance, its mass would be k times the gram molecular mass.

 

if the number of moles per litre is some number(whole or fraction) k, so now what would be the mass?

 

the after you find that out, you know that that amount would be presen in 1litre, so how much in n litres?

[DustyMcAwesome]

Ok, so I determined that Ca(NO3)2 is 164g/mol.

So m1=1.00m; v1=?; m2=0.125m; and v2=3l...... Correct?

[Fuzzwood]

Stop trying to fit all but questions 4, 5 and 6 in that m1v1=m2v2. Its completely irrelevant for questions 1, 2 and 3 because they only have 1 volume given. Again: what unit does molarity have? And what does it mean exactly?

[DustyMcAwesome]

Well what formula can I use for the other 3? I'm very confused.....

[mooeypoo]

I was trying to show you that even if you don't remember the formula, you can figure out what to do if you remember the units (which you really have to in sciences, and I assume your professor would want to see it in the test too):

 

So, knowing that Molarity = mol/Liter, the question here gets relatively easy:

 

1. how would you make 3.00L of a 0.125M solution of Ca(NO3)2?

Translate it to what you already know. You have volume = 3.00L (given).

 

You know you want to make 0.125M solution, which is mol/Liter, right?

 

M = mol/Liter

 

[math]0.125 \text{M} = \frac{\text{x mol}}{\text{2.00 Liter}}[/math]

 

And you can get the number of mols that you need from the above. Then you can do whatever you want with it - keep it as mols, or convert it to grams by molar mass, etc etc.

 

So, my point, is that if you go by the units, you can figure things out rather easily. You can repeat this for most of your wquestions above, without remembering any complicated formulas or any variety of many formulas. This is it. You have Molarity, remember what molarty is (and you kinda have to, specially when you also have 'molality' that is also a concentration but is different from molarity by units).

 

~moo