A solution of NaOH contains 0.04g of NaOH per litre. Its pH is:

Answer

To find the pH of a solution of NaOH that contains 0.04 g of NaOH per liter, we can follow these steps:

### Step 1: Calculate the molarity of NaOH
1. **Determine the molecular mass of NaOH**: The molecular mass of sodium hydroxide (NaOH) is approximately 40 g/mol.
2. **Convert grams to moles**: We can calculate the number of moles of NaOH in 0.04 g using the formula:
\[ \text{Moles of NaOH} = \frac{\text{mass (g)}}{\text{molecular mass (g/mol)}} \]
\[ \text{Moles of NaOH} = \frac{0.04 \text{ g}}{40 \text{ g/mol}} = 0.001 \text{ mol} \]

3. **Calculate molarity (M)**: Since the solution is 1 liter (1000 mL), the molarity (M) is:
\[ \text{Molarity} = \frac{\text{moles of solute}}{\text{volume of solution (L)}} \]
\[ \text{Molarity} = \frac{0.001 \text{ mol}}{1 \text{ L}} = 0.001 \text{ M} = 10^{-3} \text{ M} \]

### Step 2: Calculate the pOH of the solution
1. **Find the concentration of hydroxide ions (OH⁻)**: For NaOH, which is a strong base, it dissociates completely in solution:
\[ \text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^- \]
Therefore, the concentration of OH⁻ ions is the same as the molarity of NaOH, which is 0.001 M or \(10^{-3} \text{ M}\).

2. **Calculate pOH**: The pOH is calculated using the formula:
\[ \text{pOH} = -\log[\text{OH}^-] \]
\[ \text{pOH} = -\log(10^{-3}) = 3 \]

### Step 3: Calculate the pH of the solution
1. **Use the relationship between pH and pOH**: We know that:
\[ \text{pH} + \text{pOH} = 14 \]
Therefore, we can find the pH:
\[ \text{pH} = 14 - \text{pOH} = 14 - 3 = 11 \]

### Final Answer
The pH of the NaOH solution is **11**.

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