Cliffs AP Chemistry, 3rd Edition

• Kb= 5.02°C/m
  Restatement: Calculate MW of hydrocarbon.

∆T=Kb⋅ m

/

T

K

kg solvent

gMW

∆

b

=

^ h_ i

T

K

MW

kg solvent

grams solute

∆

b

=

^ _

^ _

h i

h i

...

./.

MW ./

C C kg solvent

C kg solvent moles solute g

gmol

78 7 76 8 0 135

502 436

852

:

• 
  

= %% =

%

``

` _

jj

j i

(d) Restatement: Determine molecular formula of hydrocarbon.

.

.

/

/

g mol empirical mass

g mol molecular mass

14 03

85

= 61

_

^

i

h

Thus, the molecular mass is 6 times greater than the empirical mass.

molecular formula =C 6 H 12

2. An experiment is to be performed to determine the formula mass of a solute (KNO 3 )
   through boiling-point elevation.

(a) What data are needed to calculate the formula mass of the solute? Create

appropriate data that can be used in part (c).

(b) What procedures are needed to obtain these data?

(c) List the calculations necessary to determine the formula mass; use your data to

calculate the formula mass.

(d) Calculate the % error in your determination of the formula mass of KNO 3 and

account for possible error(s).

Answer

2. (a)Restatement: Data needed to determine FM through B.P. elevation.

•Kb= 0.52°C ⋅ m–1

• Boiling point of water: 100.0°C


• Boiling point of KNO 3 solution: 102.2°C


• Changes in temperature between water and KNO 3 solution: 2.2°C


• Grams of solute: 10.0 g


• Grams of solvent: 50.0 g
  (b)Restatement: Procedures needed.


• Measure 50.0 grams of distilled water into a 125-mL Erlenmeyer flask.

Solutions