Please Help! A solution is made by dissolving 15.5 grams of glucose (C6H12O6) in 245 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

aaronq · Answer

use the formula: $\Delta T=i*m*K_{f}$ More on this: http://openstudy.com/study#/updates/521ecb3ae4b0750826e0c362

anonymous · Answer

Thanks!

aaronq · Answer

no problem !

anonymous · Answer

OK So I looked at the other problem but im still confused. could you walk me through the steps with this problem?

anonymous · Answer

@aaronq

aaronq · Answer

sure. first you have to find the molality, can you do that?

         $molality=\dfrac{moles ~of~solute}{kg~of ~solvent}$

anonymous · Answer

What part is the Molaity? i know it shold be $$\frac{ 15.5 }{ 245 }$$

anonymous · Answer

does that make the molaity =.0632?

aaronq · Answer

molality is m. 

you have to convert the mass of glucose to moles first, as well as converting g to kg.

anonymous · Answer

oh ok
$$\frac{ .086 m}{ .245kg }$$
Is this right

anonymous · Answer

and that = .35?

aaronq · Answer

yeah, except 0.086 is in moles, not m

now you can just multiply it by the $K_f$, to get $\Delta T $.

anonymous · Answer

$$DeltaT= (1)(0.35)(1.86)=.65$$

anonymous · Answer

so the freezing point would be -.65?

anonymous · Answer

can you help me with another question?

anonymous · Answer

A solution is made by dissolving 2.5 moles of sodium chloride (NaCl) in 198 grams 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.

aaronq · Answer

yep, because the normal freezing point is zero celsius, it is now -0.65 celsius

aaronq · Answer

This question is very similar to the link i posted, nonetheless i can help you through it.

First find the molality

anonymous · Answer

$$\frac{ 2.5M }{ .198kg }$$

anonymous · Answer

12.62m

aaronq · Answer

now multiply it by $K_b$.

Then you have to determine $i$, the van't hoff constant.

aaronq · Answer

This is the number of particles (whether it's ions or polyatomic ions) that the substance separates into when dissolved in water. so, you kinda have to know a little about their behaviour when in water.

anonymous · Answer

and the kb is .51?

aaronq · Answer

yes

aaronq · Answer

So for i,  if you had $MgCl_2\rightarrow, Mg^{2+}+2Cl^-$, i=3

anonymous · Answer

So its 6.4 but im a little confused now. what is the equation above?

aaronq · Answer

it's an example of how to determine what number for $i$ you need to use

anonymous · Answer

Oh ok so is the answer 106.4C?

aaronq · Answer

what value for $i$ did you use?