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Isotope

% Abundance

Atomic Mass

50Cr

4.345

49.946

52Cr

51.940

83.789

53Cr

9.501

52.940

54Cr

2.365

53.939

Avg. Mass = weighted average of the isotopes = [0.04345(49.946 amu)] + [0.83789(51.940 amu)]

  • +

    [0.09501(52.940 amu)] + [0.02365(53.939 amu)] = 51.996 amu

Clark College

Chemistry 100

7. Calculate the average molar mass for Chromium, Cr, from the isotopic data given.

Isotope

% Abundance

Atomic Mass

10B

19.61

10.013

11B

80.39

?

8. Only two isotopes of boron (B) are naturally occurring, their relative abundances are given in the table below. If the average atomic mass of boron is 10.811, determine the atomic mass of boron-

11.

10.811 = [10.013 amu(0.1961)] + [x amu(0.8039)]

x = 11.01 amu

9. Predict the charge for ions from the following elements, based on their positions on the periodic

table.

a) S, sulfur.

Group 6 – 8 = -2

b) Br, bromine.

Group 7 – 8 = -1

c) Ba, barium.

Group 2 = +2

d) Ga, gallium.

Group 3 = +3

Chapter 4 Homework

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