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The effect of dissolved substances on the freezing point of solutions.

Submitted by rulzdapool01 on Sun, 02/25/2007 - 10:22

Methanol (CH3OH)
Ethanol (CH3CH2OH)
Sugar (C12H22O11)
Salt (NaCl)

A cooling system has a capacity of 8L total and you want to protect your car to -40 degrees celcius. Which solute is the best choice? Why? Calculate the mass of the chosen solute that would be needed to make 8L of coolant solution for you car.

Well I assume you must be studying colligative properties. The biggest problem with your problem is that the solvent is not stated. We will assume you mean using water as a solvent. The constant K is different depending on what solvent you are using.

You are looking for a freezing point depression.

Adding a solute will cause a freezing point depression or a boiling point elevation, according to  delta T  =  K • m  where m is molality NOT molarity. For freezing point depression of water , K is 1.86°C/m

You should be concerned only with the number of moles of solute particles and not the chemical species.

Methanol, ethanol, and sugar do not dissociate in water. NaCl does dissociate. So for everyone 1 mole of NaCl you add you get two moles of solute Na+ and Cl- particles.

Water had a freezing point of 0 degrees celsius and so you need a change in temperature , or delta T, of -40 degrees.   

so you have -40 = 1.86 °C/m • m 

solve for m which will give you molality.

molality is moles of solvent per kg of solvent but since we are using water and the density of water is 1.00 g/mL  1 kg of water is = 1 L of water and molality is apprx the same as molarity.

If you need help from here let me know.

so does that mean that salt is the best choice? and how would you calculate the mass of this?

Well once you calculate the molality that is required, you multiply it by the kg of solvent.

Basically take the volume given and multiply by the denisty but since the solvent is water the molality and molarity will be the same

take m and multiply it by the volume in Liters. This will give you the number of moles of solute particles needed.  Divide this by two will tell you how many moles of NaCl you need. then multiply this by the formula weight of NaCl.