In order to make a solution that freezes at -10 degrees celsius, how many grams of NaCl must you add to 3.0 kg?

Expert Answers

An illustration of the letter 'A' in a speech bubbles

When a compound is dissolved in water, the freezing point of the solution decreases to below that of pure water. The decrease in temperature can be found by using the following equation:

T = Kf * m * i, where T is the change in the freezing point that is noticed, Kf is known as the cryoscopic constant which is a constant for water irrespective of the solute and is equal to 1.853 K*kg/mol, m is the molality of the solute and i is called the van't Hoff factor which for NaCl is 2.

The freezing point has to be decreased by 10 degree Celsius as the normal freezing point of pure water is 0 degree Celsius. We need to determine the weight of salt that has to be added to 3 kg of water.

10 = 1.853 * m * 2

=> m = 10/ (2* 1.853)

=> m = 2.698

The required molality is 2.698 or we need to add 2.698 moles of NaCl to every kilogram of water. 3 kg of water requires the addition of 3*2.698 = 8.094 moles. The mass of one mole of NaCl is 58.443 g. The mass of 8.094 moles is 58.443*8.094 = 473.09 g

We need to add 473.09 g of NaCl to 3 kg of water to reduce the freezing point to -10 degree Celsius.

See eNotes Ad-Free

Start your 48-hour free trial to get access to more than 30,000 additional guides and more than 350,000 Homework Help questions answered by our experts.

Get 48 Hours Free Access
Approved by eNotes Editorial Team