Question

what mass of water must be added to 385g of formaldehyde to prepare a 35% of solution of formaldehyde? (hint: 35% means 100g solution for 35g of solute; and solution = solute + solvent).

1. 385g of water
2. 1100g of water
3. 715g of water
4. 637.5g of water

Explanation:

Step1: Define variables

Let \( m_{\text{solution}} \) be the mass of the solution, \( m_{\text{solute}} = 385\,\text{g} \) (formaldehyde), and the mass percentage of solute is \( 35\% \). The formula for mass percentage is \( \text{Mass}\,\% = \frac{m_{\text{solute}}}{m_{\text{solution}}} \times 100\% \).

Step2: Calculate total solution mass

From \( 35\% = \frac{385\,\text{g}}{m_{\text{solution}}} \times 100\% \), solve for \( m_{\text{solution}} \):
\( m_{\text{solution}} = \frac{385\,\text{g} \times 100\%}{35\%} = \frac{38500}{35} = 1100\,\text{g} \).

Step3: Find mass of water (solvent)

Since \( m_{\text{solution}} = m_{\text{solute}} + m_{\text{solvent}} \), then \( m_{\text{solvent}} = m_{\text{solution}} - m_{\text{solute}} \).
Substitute values: \( m_{\text{solvent}} = 1100\,\text{g} - 385\,\text{g} = 715\,\text{g} \).

Answer:

1. 715g of water