Question

Salt in crude oil must be removed before the oil undergoes processing in a refinery. The

crude oil is fed to a washing unit where freshwater fed to the unit mixes with the oil and

dissolves a portion of the salt contained in the oil. The oil (containing some salt but no

water), being less dense than the water, can be removed at the top of the washer. If the

“spent” wash water contains 15% salt and the crude oil contains 5% salt, determine the

concentration of salt in the “washed” oil product if the ratio of crud oil (with salt) to

water used is 4:1.​

Answer 1

Answer:

$\large \boxed{0.64 \, \%}$

Explanation:

Assume you are using 1 L of water.

Then you are washing 4 L of salty oil.

1. Calculate the mass of the salty oil

Assume the oil has a density of 0.86 g/mL.

$\text{Mass of oil} = \text{4000 mL} \times \dfrac{\text{0.86 g}}{\text{1 mL}} = \text{3440 g}$

2. Calculate the mass of salt in the salty oil

$\text{Mass of salt} = \text{3440 g} \times \dfrac{\text{5 g salt}}{\text{100 g oil}} = \text{172 g salt}$

3. Calculate the mass of salt in the spent water

$\text{Mass of salt} = \text{1000 g water} \times \dfrac{\text{15 g salt}}{\text{100 g water}} = \text{150 g salt}$

4. Mass of salt remaining in washed oil

Mass = 172 g - 150 g = 22 g  

5. Concentration of salt in washed oil

$\text{Concentration} = \dfrac{\text{22 g}}{\text{3440 g}} \times 100 \, \% = \mathbf{0.64 \, \%}\\\\\text{The concentration of salt in the washed oil is \large \boxed{\mathbf{0.64 \, \%}} }$