# Concentration, how to determine grams of NaCl needed

Disclaimer: It has been a very long time since I have done any chemistry.

My friend posted this question on social media, I've copy+pasted it here:

Exactly $16.0~\mathrm{ml}$ of a solution A is diluted to $300~\mathrm{mL}$, resulting in a new solution B that has $0.50\ \mathrm{M}$ concentration. If the solution was made with NaCl, determine the number of grams of NaCl needed to make $1.00\ \mathrm{L}$.

It seems to me that the first part of the question is irrelevant? And all that is required here is to calculate how many grams make up $1~\mathrm{mol}$ of NaCl.

Am I missing something?

• Are you sure that this is a right question? The first part of the question is not related to second part. and also, if this is a tricky question, second part needs to be complete. Second part of the question must tell us about the concentration of that 1L solution for which we need to find mass of NaCl required..!! – ashu Apr 22 '14 at 12:28
• either you copied this question wrong or this question is really wrong..!! :) – ashu Apr 22 '14 at 12:29
• According to my friend it was copied word-for-word. – Cameron Ball Apr 22 '14 at 13:05
• I assumed that when it said "if the solution was made with NaCl" that "the solution" was referring to the 0.5M solution. – Cameron Ball Apr 22 '14 at 14:30
• I expect this question needs to read "... needed to make 1.00 L of solution A." This makes the first part of the question relevant. – Ben Norris Apr 22 '14 at 15:25

Pretty sure this is asking "How many grams of NaCl are needed to make 1.00 L of solution A?" The concentration of A is:

$$[NaCl]_A = .50 \;M * \frac{300 \; mL}{16 \; mL}$$

To make 1 L of this,

$$n_{NaCl} = [NaCl]_A * 1.00 \; L$$ $$m_{NaCl} = n_{NaCl} * MW_{NaCl}$$

The number of grams of NaCl necessary to make a liter of 0.5 molar solution is wholly determined just for that. The solvent need not be water. "If the solution were made with NaCl" Subjunctive case is required to indicate speculation.

One suspects the question asked how many mls of solution A/liter makes up 0.5 M NaCl.

• +1 for explanation of the use of the subjunctive mood (as well as deducing the most likely correct form of the question). – Cohen_the_Librarian Apr 22 '14 at 19:08
• "8^>) English is a powerful language. Don't lose the magic. – Uncle Al Apr 24 '14 at 22:19