# Molar Concentration

## Homework Statement

How many grams of NaCl (molecular weight=58.5g mole^-1) would you dissolve in water to make a 0.5M NaCl solution with 500mL final volume?

## The Attempt at a Solution

Since, 1 L of 58.5g NaCl is equal 1M I used this to firstly convert 500ml into liters which is 0.5L and then I said that half 0f 58.5g is 27.25g NaCl and this will give me 0.5 M in 0.5L of water.

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Borek
Mentor
No, 0.5 mole in 0.5L is not 0.5M.

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So, does the number of moles in NaCl change depending on the amount of water?

So, does the number of moles in NaCl change depending on the amount of water?

[STRIKE]Yes.[/STRIKE] (Misread that "moles in NaCl"...)
Here's one way to look at it:

0.5 M NaCl = 0.5 mol NaCl/1 L solution = ? mol NaCl/0.5 L solution

For the two fractions to be equal, what number does the numerator have to be?

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Borek
Mentor
So, does the number of moles in NaCl change depending on the amount of water?

There is no such thing as "number of moles in NaCl". There is either "number of moles of NaCl" or "number of moles of NaCl in the given volume of the solution".

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