A student determines the freezing point of a solution of 2.07 g of naphthalene i

Question

A student determines the freezing point of a solution of 2.07 g of naphthalene in 27.00 g of paradichlorobenzene. they gave a set of temp-time readings and I had to graph them. the time readings were to 1.0 minutes and the temp readings were also to 1.0 C.

Then I found the Freezing point of the solution to be 49.1 C.

a.) what is the freezing point depression, Delta T, given that the freezing point for pure paradichlorbenzene is 53.0 C?

I got 3.9 C by subtracting 53-49.1

b.) using Kf = 7.10 C/m(C kg/mole) for Paradichlorobenzene and m = delta T / Kf, calculate the molarity of the naphthalene.

I got .55 m

c.) using MM= Kf(number grams solute)/((number kg solvent)(delta T)) calculate the MM of naphthalene

I got ~ 140 g/mol

d.) if part c consistent with the real molar mass of 128 g/ mol?

e.)calculate the percent error that is introduced in the calculated molecular weight, if a 1.0 C error exists in the determination of the delta T? be very specific. show the calculation for 1.0 C higher and 1.0 C lower.

I got 9.4 % ( (140-128)/128 x 100 = 9.4 %)

if + 1.0C i got 13.2 %

if - 1.0 C i got 46.6 %

f.) how accurately must we determine delta T in order to be assured of an accurate value for the molecular weight? be very specific by supplying a numerical value. explain your rational.

g.) what changes or refinements in the experimental procedure do you recommend to attain the desired accuracy in delta T? be very specific,

I AM SO SORRY THIS IS SO LONG! PLEASE COMFIRM THAT I HAVE BEEN DOING THIS RIGHT. IF I AM NOT, PLEASE SHOW STEPS. I REALLY JUST NEED HELP ON F AND G, BUT I PUT THE REST THERE SO YOU CAN SEE MY ANSWERES. THANK YOU SO MUCH!!!!! :)

Explanation / Answer

f.) how accurately must we determine delta T in order to be assured of an accurate value for the molecular weight? be very specific by supplying a numerical value. explain your rational.

When delta T = 3.9 C,

MM= Kf(number grams solute)/((number kg solvent)(delta T)) = 7.10*2.07/(0.027*3.9) = 139.57 g/mol

Error = |(128 - 139.57)|/128 *100 = 9%

When delta T = 4.9 C,

MM= Kf(number grams solute)/((number kg solvent)(delta T)) = 7.10*2.07/(0.027*4.9) = 111.1g/mol

Error = (128 - 111.1)/128 *100 = 13.2 %

Since the actual molecular weight is 128 g/mol which lies between 111.1g/mol and 139.57 g/mol, the value of delta T should be between 3.9 and 4.9 to get the most accurate result.

Lets try, delta T = 3.9 + 0.5 = 4.4 C,

MM= Kf(number grams solute)/((number kg solvent)(delta T)) = 7.10*2.07/(0.027*4.4) = 123.7 g/mol

Error = |(128 - 123.7)|/128 *100 = 3.4% ( which is much near to the actual value but actually is a bit less)

So, Lets try delta T = 3.9 + 0.3 = 4.2 C,

MM= Kf(number grams solute)/((number kg solvent)(delta T)) = 7.10*2.07/(0.027*4.4) = 129.6 g/mol

Error = |(128 - 129.6)|/128 *100 = 1.25% ( which is much near to the actual value)

So, delta T should be around 4.2 C to get must accurate value of molecular weight.

g.) what changes or refinements in the experimental procedure do you recommend to attain the desired accuracy in delta T? be very specific,

1. Thermometer that gives more precise reading should be used to measure the temperature of the solution.

2. More sets of readings can be taken to reduce the possible error due to room temperature/volatile nature of naphthalene and paradichlorobenzene.

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