# Question 5c41e

Sep 22, 2014

You would have to add 6 g of succinic acid or 9 g of caffeine to increase the boiling point by 3 °C. To decrease the freezing point by 2 °C, you would have to add 3 g of succinic acid or 5 g of caffeine.

In your problem, we have to work backwards from the boiling point elevation or freezing point depression to calculate the mass of solute.

You have given us four problems. Here's how to answer the boiling point problem with succinic acid.

Step 1. Calculate the molality of the solution.

ΔT_"b" = iK_"b"m

m = (ΔT_"b")/(iK_"b") = "3 °C"/("1 × 6.2 °C·kg·mol"^-1)# = 0.48 mol·kg⁻¹ (1 significant figure + 1 guard digit)

Step 2. Calculate the moles of succinic acid

Moles of SA = 0.100 kg × $\text{0.48 mol"/"1 kg}$ = 0.048 mol

Step 3. Calculate the mass of SA

The formula of SA is C₄H₆O₄. The molar mass is 118.09 g·mol⁻¹.

Mass of SA =0.048 mol SA × $\text{118.09 g SA"/"1 mol SA}$ = 6 g SA

Note: The answer can have only 1 significant figure, because that is all you gave for the boiling point elevation. If you need more precision, you will have to recalculate.

You can use the above model to show that

• It takes 9 g of caffeine to raise the boiling point by 3 °C.
• It takes 3 g of succinic acid to lower the freezing point by 2 °C.
• It takes 5 g of caffeine to lower the freezing point by 2 °C.