5 Steps to a 5 AP Chemistry

 Answers and Explanations

a. The percent oxygen (53.3%) is determined by subtracting the carbon and the hydrogen

from 100%.

For C: 40.0/12.01 =3.33 Divide each C = l

For H: 6.71/1.008 =6.66 of these by H = 2

For O: 53.3/16.00 =3.33 the smallest O = 1

(3.33)

This gives the empirical formula: CH 2 O.

You get 1 point for correctly determining any of the elements, and 1 point for

getting the complete empirical formula correct.

b. Using HA to represent the monoprotic acid, the balanced equation for the titration

reaction is:

HA +NaOH →NaA +H 2 O

The moles of acid may then be calculated:

(45.00 mL NaOH)(0.1000 mol NaOH/1000 mL) (1 mol HA/1 mol NaOH) =

4.500 × 10 −^3 mol HA

The molecular mass is:

0.2720 g/4.500 × 10 −^3 mol =60.44 g/mol

You get 1 point for the correct number of moles of HA (or NaOH) and 1 point for

the correct final answer.

c. This may be done in several ways. One way is to use the ideal gas equation. This will

be done here. The equation and the value of Rare given in the exam booklet.

First find the moles: n=PV/RT Do not forget, you MUST

change to kelvin.

n=(1.00 atm)(300.0 mL)(l L/1000 mL)/(0.0821 L atm/mol K)(373 K)

n=9.80 × 10 −^3 mol

The molecular mass is: 1.18 g/9.80 × 10 −^3 mol =120 g/mol

You get 1 point for getting any part of the calculation correct and 1 point for

getting the correct final answer.

d. The approximate formula mass from the empirical (CH 2 O) formula is:

12 +2(1) + 16 =30 g/mol

For part b: (60.44 g/mol)/(30 g/mol) = 2

Molecular formula = 2 ×Empirical Formula =C 2 H 4 O 2

For part c: (120 g/mol)/(30 g/mol) = 4

Molecular formula = 4 ×Empirical Formula =C 4 H 8 O 4

You get 1 point for each correct molecular formula. If you got the wrong answer in

part a, you can still get credit for one or both of the molecular formulas if you used the

part avalue correctly.

100  STEP 4. Review the Knowledge You Need to Score High