# Chemical Forums

## Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: john4614 on November 02, 2008, 03:56:21 PM

Title: Molarity and Mass %
Post by: john4614 on November 02, 2008, 03:56:21 PM
Hello everyone, some pointers for this problem please. No sure of number 1, im sure if i get #1 i can get #2 hopefully, please explain answers.
1) 20.04g sample of NaCl is dissolved in enough water to prepare 100.0g of solution. The density of the solution is 1.122g/mL. What is the molarity of the salt solution?
2) For the solution described above, determine the mass percent of the solution present.
M= moles of solute
Liter Of solution (mol/L)

Mass%=mass component   x 100%
Total mass sample

Title: Re: Molarity and Mass %
Post by: Arkcon on November 02, 2008, 04:09:45 PM
Can you plug some of your given values into the formulas you have?  *HINT* if the value you're given has units that match that part of the formula, that's where it goes.  If not, you may have to convert units, with a formula you were given elsewhere, or on a quiz, some time ago.
Title: Re: Molarity and Mass %
Post by: john4614 on November 02, 2008, 08:32:43 PM
20.04 gram has to convert into Moles correct? is this done by 6.023x10^23?
Title: Re: Molarity and Mass %
Post by: john4614 on November 05, 2008, 06:42:30 PM
okay this is due in 24hours so please help me, please check my work...no idea if i did this correctly:
1)

20.04g= 1 molNaCL   = .7157
28gNaCl

.7157molNaCl= .6379M
.001122L

2)
moles Nacl   = 20.04   x   1NaCl= .7156
100g Sol       100g sol         28   100gSolution

100gx 1mL
1.122g/mL=  0.08913L

molarity= molesNaCl   =   .6379mol NaCl   =   7.16M
litter of solution     0.08913L
Title: Re: Molarity and Mass %
Post by: Borek on November 06, 2008, 03:36:37 AM
Molar mass of NaCl is not 28 g.

Volume of the solution is not 0.001122L. Check your units and you will see that something is wrong.

You have calcualted correctly volume of the solution looking for the answer to the second part of the question.

Second part of the question doesn't require you to calculate moles. Look at the definition of percentage concentration - which of the values needed are you directly given in the question?
Title: Re: Molarity and Mass %
Post by: john4614 on November 06, 2008, 04:00:41 PM
NaCl=58.4425 g/mol ? Is everything else correct if i plug that in?
Title: Re: Molarity and Mass %
Post by: Borek on November 06, 2008, 05:14:32 PM
No, I have already listed other problems.
Title: Re: Molarity and Mass %
Post by: john4614 on November 06, 2008, 07:30:25 PM
okay Im confussed of what to put because it gives me 1.122g/mL but for the formula it ask the liter of the solution...I know that to get mL to liters you move the decimal 3 times. but i dont have just mL i have g/mL.. Looked in book can't find the answer im looking for.
Title: Re: Molarity and Mass %
Post by: DrCMS on November 06, 2008, 08:24:14 PM
Start from the beginning again.

What is the definition of molarity?
What is the definition of density?

You have a weight of NaCl in 100g of solution of known density?

What is the volume of that solution?

So using the correct mass of NaCl you can calculate the molarity

Part 2 is just simple maths from the weights given.
Title: Re: Molarity and Mass %
Post by: john4614 on November 07, 2008, 12:18:05 AM
okay i try and try to get it not sure if this is correct. I have been trying over and over. If i dont get this someone tell me what numbers are wrong and what should be there please!
1)
20.04      1Mol NaCl
------  X ----------= 17.861m
1            1.122g/mL

2)
100g
-----  x100%= 5.59879=== 5.5%
17.861

Correct incorrect????  ???
Title: Re: Molarity and Mass %
Post by: Borek on November 07, 2008, 03:12:14 AM
For the first problem you are asked to calculate molar concentration.

Molar concentration is number of moles of solute/volume of the solution

You have correctly calculated number of moles of NaCl.

You don't have solution volume yet.

You are told you have 100g of solution of density 1.122 g/mL.

What is its volume?