C. Johannesson

Ch. 4 - Molar Relationships

III. Percent Composition(p. 143-144)

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A. Definition

the percent by mass of each element in a compound

100mass total

element of massncompositio %

to distinguish between similar compounds to determine elemental mass in a sample

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100 =

B. Calculation of % Comp

Find the % composition of FeO.

%Fe =55.85 g

71.85 g 100 =

%O =16.00 g

71.85 g

77.73% Fe

22.27% O

C. Johannesson

100 =

B. Calculation of % Comp

Find the % composition of Fe2O3.

%Fe =111.70 g

159.70 g 100=

%O =48.00 g

159.70 g

69.944% Fe

30.06% O

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C. Compound Identification A compound contains 28 g Fe and 8.0 g O. Is it

FeO or Fe2O3?

%Fe =28 g

36 g 100 =

The compound is FeO.

78% Fe

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D. Elemental Mass in a Sample How many grams of iron are in a

38.0-gram sample of iron(III) oxide?

38.0 g Fe2O3 69.944 g Fe

100 g Fe2O3 = 26.6 g Fe

Fe2O3 is 69.944% Fe

- or -

(38.0 g Fe2O3)(0.69944) = 26.6 g Fe

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CHALLENGE PROBLEM

A mining company needs to choose a mine site. Which would provide a better source of iron? Ore containing 32% FeO or ore containing 48% Fe2O3?

FeO is 77.73% Fe

Fe2O3 is 69.944% Fe

SOLUTION

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100 =

E. % Water in a Hydrate

CaCl2·2H2O

calcium chloride dihydrate

%H2O =36.04 g

147.02 g 24.51%

H2O

100mass total

waterof massOH % 2