# Homework Help: Question related to molarity

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1. Jun 24, 2015

### Prashasti

Ammonia gas is passed into water, yielding a solution of density 0.93 g/cm^3 and containing 18.6% NH3 by weight. The mass of NH3 per cc of the solution is
1) 0.17
2) 0.51
3) 0.34
4) 0.68

I solved it using the molarity equation.

Molarity = 10xd/MM

where,

x - % by weight
d - density
MM - Molar mass

And then I multiplied molarity with Molar mass (17 g mol^-1) and got 172.98 as the answer, whereas the answer is 0.17.

Where did I go wrong?

2. Jun 24, 2015

### Staff: Mentor

Here.

First, this equation is wrong as written, second, even in the correct form it is not universal - it works only in specific situations. Do you know what they are?

3. Jun 24, 2015

### Prashasti

No. I'm in high school. So, I don't know much about that. What specific situations are you talking about? .

4. Jun 24, 2015

### Staff: Mentor

You were given this equation without any explanation?

5. Jun 27, 2015

### theodoros.mihos

Let $x,y$ the number of moles in a cc. Writing the problem we have:
$$\frac{17x}{17x+18y} = 0.186\times 0.93$$

6. Jun 27, 2015

### Staff: Mentor

Perfect way of overcomplicating things. How you are going to solve one equation with two unknowns?

7. Jun 29, 2015

### epenguin

Forget about molarity which doesn't enter into his simple arithmetical problem.

Even forget about the density - it's not a million miles away from 1 g/cc.

Then only one of the given answers is anywhere near anything you would expect.

Though once you have seen which it is, do the working out of the exact answer.

Last edited: Jun 30, 2015
8. Jul 5, 2015

### mooncrater

HINT:
MOLARITY is the number of moler PER LITRE of solution. Whereas you are asked the mass of NH3 in millilitres (or cc).