An aqueous solution of chromium(II) sulfate has a concentration of 0.143 molal. The percent by mass of chromium(II) sulfate in the solution is %.
The correct answer and explanation is:
To find the percent by mass of chromium(II) sulfate (CrSO₄) in a solution with 0.143 molal concentration, we follow these steps:
Step 1: Understand Molality
Molality (m) is defined as: molality=moles of solutekilograms of solvent\text{molality} = \frac{\text{moles of solute}}{\text{kilograms of solvent}}
So 0.143 molal means: 0.143 mol of CrSO₄ per 1 kg (1000 g) of water0.143 \text{ mol of CrSO₄ per 1 kg (1000 g) of water}
Step 2: Calculate the Mass of CrSO₄
First, calculate the molar mass of chromium(II) sulfate (CrSO₄):
- Cr = 51.996 g/mol
- S = 32.06 g/mol
- O₄ = 4 × 16.00 = 64.00 g/mol
Molar mass of CrSO₄ = 51.996 + 32.06 + 64.00 = 148.056 g/mol
Now, calculate the mass of 0.143 mol CrSO₄: Mass of CrSO₄=0.143 mol×148.056 g/mol=21.174g\text{Mass of CrSO₄} = 0.143 \, \text{mol} \times 148.056 \, \text{g/mol} = 21.174 g
Step 3: Total Mass of Solution
Total mass = mass of solute + mass of solvent Total mass=21.174 g+1000 g=1021.174 g\text{Total mass} = 21.174 \, \text{g} + 1000 \, \text{g} = 1021.174 \, \text{g}
Step 4: Calculate Percent by Mass
Percent by mass=(mass of solutetotal mass of solution)×100\text{Percent by mass} = \left( \frac{\text{mass of solute}}{\text{total mass of solution}} \right) \times 100 Percent by mass=(21.1741021.174)×100≈2.074%\text{Percent by mass} = \left( \frac{21.174}{1021.174} \right) \times 100 \approx 2.074\%
✅ Final Answer:
The percent by mass of chromium(II) sulfate is approximately 2.07%.
Explanation (300 words)
Percent by mass is a way of expressing the concentration of a component in a mixture or solution. It tells us what fraction of the total mass of the solution is due to the solute—in this case, chromium(II) sulfate (CrSO₄).
We’re given the molality of the solution as 0.143 mol/kg, which means 0.143 moles of CrSO₄ are dissolved in 1 kilogram (or 1000 grams) of water. To calculate the mass of the dissolved CrSO₄, we need its molar mass. By summing the atomic masses of chromium (Cr), sulfur (S), and four oxygen (O) atoms, we get 148.056 g/mol.
Multiplying 0.143 mol by the molar mass gives us the mass of the solute: approximately 21.174 grams. To find the percent by mass, we add this to the mass of the water (1000 g) to get the total mass of the solution: 1021.174 grams. The final step is to divide the mass of the solute by the total mass of the solution and multiply by 100 to convert to a percentage.
This results in a final concentration of about 2.07% by mass. This value is useful in laboratory settings where precise concentrations are needed for chemical reactions, as it directly relates to the weight of substances, which is often easier to measure than volume or moles.