Page 56 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 3] ATOMS AND ATOMIC MASSES 45

3.4. Which part of Problem 3.3 illustrates the Law of Multiple Proportions?
Ans. Part (b).
0.734 1.50 3
= =
0.488 1.00 2
3.5. Why was Dalton’s contribution different from that of the ancient Greeks who postulated the existence of
atoms?

Ans. Dalton based his postulates on experimental evidence; the Greeks did not. Also, he postulated that atoms of
different elements are different.

3.6. Could anyone have done Dalton’s work 100 years earlier than Dalton did?
Ans. No; the laws on which his theory was based had not been established until shortly before his work.

3.7. If 20 kg of coal burns, producing 100 g of ashes, how is the Law of Conservation of Mass obeyed?
Ans. The coal plus oxygen has a certain mass. The ashes plus the carbon dioxide (and a few other compounds that
are produced) must have a combined mass that totals the same as the combined mass of the coal and oxygen.
The law does not state that the total mass before and after the reaction must be the mass of the solids only.

3.8. A sample of water purified from an iceberg in the Arctic Ocean contains 11.2% hydrogen by mass.
(a) What is the percent of hydrogen by mass in water prepared in the lab from hydrogen and oxygen?
(b) How can you predict that percentage?
Ans. (a) 11.2% (b) The law of definite proportions states that all water, no matter its source, contains the same
percentage of its constituent elements.

3.9. Chemists do not use the Law of Multiple Proportions in their everyday work. Why was this law introduced
in this book?
Ans. It is important in the history of chemistry, but mostly it was introduced to show that Dalton’s atomic theory
was based on experimental data.

3.10. What happens to a postulate of a scientific theory if it leads to incorrect results?

Ans. It is changed or abandoned.

ATOMIC MASSES
3.11. The average man in a certain class weighs 190 lb. The average woman in that class weighs 135 lb. The
male/female ratio is 3:2. What is the average weight of a person in the class?
Ans. Three of every five people are male. Thus:
3 2

(190 lb) + (135 lb) = 168 lb
5 5
3.12. (a) Determine the total weight of all the men and the total weight of all the women in your chemistry class
(by using an anonymous questionnaire) (b) Attempt to determine the ratio of the average weight of each
man to that of each woman without counting the numbers of each. (c) Attempt the same determination
at a dated party.
Ans. (b) You cannot determine the ratio of averages without knowing something about the numbers present.
(c) At a dated party there is apt to be a 1:1 ratio of men to women; therefore the ratio of the averages is
equal to the ratio of the total weights.

3.13. If 75.77% of naturally occurring chlorine atoms have a mass of 34.968 852 amu and 24.23% have a mass
of 36.965 903 amu, calculate the atomic mass of chlorine.