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These new mass ratio numbers are highly suggestive in thefollowing way. Notice that, in each case, we took the ratio of oxygen mass to a nitrogen mass of 1, and thatthe resultant ratios have a very simple relationship:
| Assuming that: | Oxide C is | Oxide B is | Oxide A is |
|---|---|---|---|
| Oxide A is | |||
| Oxide B is | |||
| Oxide C is |
We don't have a way (from these data) to know which of these sets of molecular formulae are right. But wecan assert that either one of them or one analogous to them is right.
Similar data are found for
any set of compounds formed from common elements. For example,there are two oxides of carbon, one with oxygen to carbon mass
ratio 1.33:1 and the other with mass ratio 2.66:1. The secondoxide must have twice as many oxygen atoms, per carbon atom, as
does the first. The general statement of this observation is the
Law of Multiple Proportions .
When two elements
combine to form more than one compound, the mass of element Awhich combines in the first compound with a given amount of
element B has a simple whole number ratio with the mass ofelement A which combines in the second compound with the same
given mass of element B.Law of multiple
proportions
In explaining our observations of the Law
of Multiple Proportions for the carbon oxides and thenitrogen oxides, we have concluded that the simple mass ratio
arises from the simple ratio of atoms contained in theindividual molecules. Thus, we have established the following
postulates of the
Atomic Molecular Theory .
Atomic molecular
theory
Assume that matter does not consist of atoms. Show by example how this assumption leads tohypothetical predictions which contradict the Law of Multiple Proportions. Do these hypothetical examplescontradict the Law of Definite Proportions? Areboth observations required for confirmation of the atomic theory?
Two compounds, A and B, are formed entirely from hydrogen and carbon. CompoundA is 80.0% carbon by mass, and 20.0% hydrogen, whereas Compound B is 83.3% carbon by mass and 16.7%hydrogen. Demonstrate that these two compounds obey the Law of Multiple Proportions. Explain whythese results strongly indicate that the elements carbon and hydrogen are composed ofatoms.
In many chemical reactions, mass does not appear to be a conservedquantity. For example, when a tin can rusts, the resultant rusty tin can has a greater mass than before rusting. Whena candle burns, the remaining candle has invariably less mass than before it was burned. Provide an explanation ofthese observations, and describe an experiment which would demonstrate that mass is actually conserved in thesechemical reactions.
The following question was posed on an exam:
An unknown non-metal element (Q) forms two gaseous fluorides of unknown molecular formula. A 3.2 g sampleof Q reacts with fluorine to form 10.8 g of the unknown fluoride A. A 6.4 g sample of Q reacts with fluorine toform 29.2 g of unknown fluoride B. Using these data only, demonstrate by calculation and explanation thatthese unknown compounds obey the Law of Multiple Proportions.A student responded with the following answer:
The Law of Multiple Proportions states that when twoelements form two or more compounds, the ratios of the masses of the elements between the two compounds are ina simple whole number ratio. So, looking at the data above, we see that the ratio of the mass of element Q incompound A to the mass of element Q in compound B is , which is a simple whole number ratio. This demonstrates that these compounds obey the Law of Multiple Proportions.Assess the accuracy of the students answer. In your assessment, you must determine what information is correct or incorrect, provide the correct information where needed, explain whether the reasoning is logical or not, and provide logical reasoning where needed.
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