Question

Predict which of the following would be the most effective anti-freeze, assuming equal weight of each are used: methyl alcohol(CH3OH) ; sucrose(C12H22O11); ethyl alcohol (C2H5OH); ethylene glycol ( C2H4(OH)2). Explain your choise!

Answer 1

To choose best anti-freeze system, consider following calculation-

Imagine now that we want to protect 4 liters of water

Knowing that the molality of ethylene glycol must be 21.5 mol / kg solvent, we can calculate the number of moles of ethylene glycol required:

4.0 L H2O = 4.0 kg H2O (at 4˚C because the density of water is 1.0 g / mL at 4˚C)

# moles ethylene glycol = m(solute) x (mass H2O)

# moles ethylene glycol = (21.5 mol / kg solvent) x (4kg H2O)

# moles ethylene glycol = 86 moles

Ok, now that we have the number of moles of antifreeze, we can calculate the mass of antifreeze required.

mass antifreeze = (# moles ethylene glycol) x (molecular mass ethylene glycol)

mass antifreeze = (86 mol) x (61.2 g / mol)

mass antifreeze = 5263.2g = 5.26 kg of antifreeze

Now that we have the mass of ethylene glycol required, we can calculate the proper % mixture by mass:

% antifreeze by mass = (mass antifreeze) % (mass antifreeze + mass water) x 100%

% antifreeze by mass = [5.26kg / (5.26kg + 4.0kg)] x 100%

% antifreeze by mass = (5.26 / 9.26) x 100%

% antifreeze by mass = 57%

Assuming the density of ethylene glycol to be 1.114 g / mL we can calculate the volume of antifreeze required to protect water to -40˚C

volume antifreeze = (mass antifreeze) x (density antifreeze)

volume antifreeze = (5263 g) / (1.114g / mL)

volume antifreeze = 4724 mL = 4.72 L of antifreeze

Knowing the volume of ethylene glycol required, we can calculate the proper % mixture by volume:

% antifreeze by volume = (volume antifreeze) % (volume antifreeze + volume water) x 100%

% antifreeze by volume = (4.72 L) / (4.72 L + 4.0 L) x 100%

% antifreeze by volume = (4.72 / 8.72) x 100%

% antifreeze by volume = 54%

So, in summation .. to protect 4 L of water to -40˚C a volume of 4.72 L antifreeze (ethylene glycol) is required. Put another way, if you mix your antifreeze by percentage, you should use roughly 50% water, 50% antifreeze!

Now for other remaining solvents, calculate it same way, you will get answer.