What is the concentration of a solution containing 2g of NaOH in 100cm3 of solution? [Na = 23, O = 16, H = 1]

Analysis

To find the concentration of the solution, we need to determine how many moles of solute (NaOH) are present in a specific volume of solution (expressed in $dm^3$).

Concept

The concentration ($C$) in $mol/dm^3$ is calculated using the formula:

Where:

• $n$ = number of moles of the solute
• $V$ = volume of the solution in $dm^3$

To find the number of moles ($n$), we use:

Step-by-Step Solution

Step 1: Calculate the molar mass of NaOH

• Na = 23, O = 16, H = 1
• Molar mass of NaOH = $23 + 16 + 1 = 40 \, g/mol$

Step 2: Calculate the number of moles ($n$) of NaOH

• Mass given = $2 \, g$
• $n = \frac{2 \, g}{40 \, g/mol} = 0.05 \, mol$

Step 3: Convert the volume from $cm^3$ to $dm^3$

• $1000 \, cm^3 = 1 \, dm^3$
• $100 \, cm^3 = \frac{100}{1000} \, dm^3 = 0.1 \, dm^3$

Step 4: Calculate the concentration ($C$)

• $C = \frac{0.05 \, mol}{0.1 \, dm^3}$
• $C = 0.5 \, mol/dm^3$

Conclusion

The calculated concentration is 0.50 mol $dm^{-3}$. This matches option B.

Exam Tip

Always remember the conversion factor: $1 \, dm^3 = 1000 \, cm^3$. A common mistake in JAMB Chemistry is forgetting to convert $cm^3$ to $dm^3$, which leads to an answer that is 1000 times larger than the correct one. Also, ensure you always calculate the molar mass correctly from the atomic masses provided in the bracket!