Question

What is the final concentration of NaCl upon mixing 10 ml of 10 mM NaCl with 990 ml of 10 mM NaCl?

• A. 0.1 mM
• B. 0.1 M
• C. 0.01 M
• D. 1.0 mM

Hint:

If both solutions have the same concentration, the final concentration remains the same. Convert between mM and M carefully to match the answer format.

Answer 1

The Correct Option is C

Step 1: Understand the given values.
Volume of solution 1: \(10\, {ml}\) of \(10\, {mM} = 0.01\, {M}\) NaCl
Volume of solution 2: \(990\, {ml}\) of \(10\, {mM} = 0.01\, {M}\) NaCl
Total volume after mixing: \(10\, {ml} + 990\, {ml} = 1000\, {ml} = 1.0\, {L}\)

Step 2: Use the concept of molarity.
Since both solutions have the same concentration, there is no dilution effect due to concentration difference. Hence:
\[ {Final concentration} = \frac{(10 + 990) \times 10^{-3} \, {mol/L}}{1.0\, {L}} = 0.01\, {M} \]

Step 3: Match the correct unit.
\(0.01\, {M} = 10\, {mM}\), but since the option listed is in M, answer (3) 0.01 M is correct.