A student completes a titration by adding 12.0 milliliters of NaOH(aq) of unknown concentration to 16.0 milliliters of 0.15 M HCl(aq).
What is the molar concentration of the NaOH(aq)?
(1) 0.11 M (3) 1.1 M
(2) 0.20 M (4) 5.0 M
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Best Answer
NaOH+ HCl--> NaCl+ H2O
From this balanced equation, we know that 1 mol NaOH= 1 mol HCl (keep in mind this because it will be used later).

We also know that 0.15 M HCl aqueous solution (soln)= 0.15 mol HCl/ 1 L of HCl soln (this one is based on the definition of molarity).

First, we should find the mole of HCl:
16.0 mL HCl soln* (1 L HCl soln/ 1,000 mL soln)* (0.15 mol HCl/ 1L HCl soln)= 2.40* 10^(-3) mol HCl.

Now, let's find the concentration of NaOH aqueous soln:
2.40* 10^(-3) mol HCl* (1 mol NaOH/ 1 mol HCl)* (1/ 12.0 mL NaOH soln)* (1,000 mL NaOH soln/ 1L NaOH soln)= 0.20 M NaOH aqueous soln.

The final answer is (2) 0.20 M NaOH (aq).

Also, this problem can also be done by using dimensional analysis. Don't forget significant figures.

Hope this would help~ 
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