A solution is made by dissolving 25.5 grams of glucose (C6H12O6) in 398 grams of water. What is the freezing-point depression of the solvent if the freezing point constant is -1.86 °C/m? Show all of the work needed to solve this problem.

Question

jfraser · Answer

turn the mass of glucose into moles, and plug into the equation for freezing-point depression$$\Delta T_f = (\frac{mol solute}{kg solvent})* K_f$$

anonymous · Answer

can you show me how to do it ? its been haunting me all day

jfraser · Answer

turn mass into moles. use the molar mass. If you're doing FP depression, you know how to turn mass into moles

anonymous · Answer

no

anonymous · Answer

can you walk me through it?

jfraser · Answer

moles = mass divided by molar mass:

$$moles = mass * (\frac{moles}{mass})$$

anonymous · Answer

can you fill it in and i can try the math, and you check it?

jfraser · Answer

i've already done the math, you tell me what you get ;-)

anonymous · Answer

1 mol, don't know what the mass is

jfraser · Answer

you've got the formula of glucose, find the molar mass using the PT

anonymous · Answer

180.09

jfraser · Answer

that's the molar mass, now find the moles of 25.5g

anonymous · Answer

how? lol :/

anonymous · Answer

?

jfraser · Answer

plug the mass and molar mass into the formula i posted above

anonymous · Answer

i dont know how. can you do it? only plug it in tho

jfraser · Answer

the mass given is 25.5g.

the molar mass is 180g.

anonymous · Answer

25.5*1/180

jfraser · Answer

yes, but that's just finding moles. those moles have to go into the equation i posted first $$\Delta T_f = (\frac{mol solute}{kg solvent})* K_f$$$$\Delta T_f = (\frac{0.142mol solute}{kg solvent})* K_f$$
now find the rest and plug in to find DT

anonymous · Answer

whats the * k

jfraser · Answer

Kf is the freezing point depression constant, and it's given

anonymous · Answer

is the kg of solvent given?

jfraser · Answer

the grams of solvent are given, turn it into kg

anonymous · Answer

C6H12O6 is this the solvent

anonymous · Answer

.398 kg

jfraser · Answer

the glucose is the solUTE that you just solved for. what's the solVENT?

anonymous · Answer

water

jfraser · Answer

the 398g of water is the solvent

now plug in the proper values and solve

anonymous · Answer

-.663

anonymous · Answer

correct?

jfraser · Answer

that's what i get

anonymous · Answer

can you check this?

anonymous · Answer

A solution is made by dissolving 4.0 moles of sodium chloride (NaCl) in 2.05 kilograms of water. If the molal boiling point constant for water (Kb) is 0.51 °C/m, what would be the boiling point of this solution? Show all of the work needed to solve this problem.

jfraser · Answer

same problem, different numbers

anonymous · Answer

can you do it and give me the final answer

jfraser · Answer

no, but i'll tell you if your answer is correct ;-)

anonymous · Answer

109 A°C
382.15 K

jfraser · Answer

too high

anonymous · Answer

imm going to do it again and show all work, can you check it?

jfraser · Answer

sure

anonymous · Answer

hbow do i get the mass given?

anonymous · Answer

molar mass is 58.6, correct?

jfraser · Answer

you don't need to use molar amss, because it already tells you the moles of NaCl. plug them in as is

anonymous · Answer

would it be 234

jfraser · Answer

4 moles of NaCL would weigh 234g, but you don't use grams in these equations, you use moles.

it tells you 4 moles of NaCl, so plug in 4 moles of NaCl

anonymous · Answer

4 moles of solute* .51C / 2.05 Kg ?

anonymous · Answer

.995 C

anonymous · Answer

274.145 K

jfraser · Answer

that's the change in the boiling point, so the actual boiling point will be?

anonymous · Answer

how do you do that
subtract the two?

anonymous · Answer

274.145- 273.66?

jfraser · Answer

what temp does water normally boil at?

jfraser · Answer

what temp does water normally boil at?

anonymous · Answer

212
373.15 k

anonymous · Answer

373.15-274.145

jfraser · Answer

the change in temp is given in C, not F or K, so keep your units straight and this will all make a whole lot more sense.

water boils at 100C.

the CHANGE in temp of the BP is 0.99C

the NEW BP of this solution will be the sum of the two, because it's always an elevation in the boiling point

anonymous · Answer

so boiling point changes by 2 degrees

jfraser · Answer

the BP changes by 1 degree. on what planet does 0.99 = 2?

anonymous · Answer

lol thats what i meant but i hit 2

jfraser · Answer

ok then

anonymous · Answer

doesnt it go down?

jfraser · Answer

BPs are always raised by solutions, FPs are always decreased by solutions

jfraser · Answer

it has to do with the interactions that the solute molecules create in between the solvent molecules. The solute gets in the way of the phase change, no matter what direction

anonymous · Answer

.01 right