Question

Calculate the molar mass of each Compound :
Gycerol
NaCl
CaCl
This is for the lab experiment on Molar Mass and freezing point, and these are the results l got.
Solution. Freezing point in degreesC
H2O. 0
Gycerol. -3.5
NaCl. -5.7
CaCl. -4.0
How do these molar masses compare to the theoretical. Calculate the percent error. glycerol I have 0.04470, and 0.0383 for NaCl and 0.01792 for CaCl. I know there is a constant l am supposed to use to get the theoretical molar mass, but l am stuck. CALCULATE THE PERCENT ERROR.

Answer 1

In this experiment you are calculating molar masses of the above mentioned compounds and comparing them with the actual values for % error.

The basis for computing the molar mass is the eqn below :
dT = - K_f*m

Here Kf is called cryoscopic constant, and m is molality , which equals moles of solute per kg of solvent

Let's say you dissolve 'x' grams of NaCl in 'y' grams of water and compute the freezing point. Let's say it comes out to be T₁⁰C

Freezing point of pure water = T = 0⁰C

Thus, dT = T-T₁ = -T₁

Putting this value in above eqn you can get value of m.

m = (x/M) / (y/1000)

Here M is molar mass of solute

Thus in this way you get the experimental value of M

Let's say the actual value of molar mass available in literature is M_a

Thus % error = abs(M_a-M) / M_a * 100

Here abs denotes the absolute value of the result (M_a-M)

NOTE : It's important to understand that in % error calculation, Ma should be in the denominator and not M , becaus error is always calculated relative to the actual value.