Nerd_Bunny wrote:Has anyone taken the University of Michigan invite test? My partner and I did the test yesterday and we were wondering if anyone was able to correctly do the math on the lab. Advice as well would be really helpful. Thanks!

Test writer here-

So let's say you have

moles of NaCl and

moles of CaCl2; we want to find

and

.

From the experiment you should get the mass

of the sample, the mass of water used

, the boiling temperature without dissolved salt

, and the boiling temperature with salt

.

The boiling point elevation formula says that

where

is the molality and

is the Van't Hoff factor (sp?), which can be approximated as the number of ions that one molecule of a substance dissociates into. In particular,

and

.

Here,

. Since the two salts have different Van't Hoff factors, their contributions need to be calculated separately;

,

and

. Since we assumed

moles of NaCl and

moles of CaCl2,

and

. So you end up with

, which looks kind of ugly but is just an equation in

and

since everything else is known.

Also, from the masses you get

, note that the

s here are mass not molality (why does chemistry use the same symbols for everything qq). So

which is also an equation in

and

.

So you just have a system of equations:

which should be solvable, making sure to account for SI units and such.

For example, if

C,

C,

kg, and

g, you have

from which you get

moles and

moles and then you can find the percent mass from there

In retrospect I probably should have given a procedure for everyone to follow at least, so that they could get to the math here because very few people ended up even considering

, or just made it a test question and had a slightly easier lab ... oops

Feel free to ask me other questions about my test if my messy typed out equations didn't make sense lol