If you analyse tap water from your home and water from an Alpine glacier, will they have the same proportion of hydrogen and oxygen? The answer is yes, always, and that very simple fact is one of the most useful laws in chemistry.
At the end of the 18th century, the French chemist Joseph Louis Proust analysed the same compound obtained from very different sources (natural minerals, laboratory reactions...) and found something surprising: the proportion between the elements that made it up was always identical.
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We analyse distilled water from a laboratory and collected, purified rainwater. In what ratio do hydrogen and oxygen combine in each? What does Proust's law say about…
We analyse distilled water from a laboratory and collected, purified rainwater. In what ratio do hydrogen and oxygen combine in each? What does Proust's law say about the origin of the samples?
If you know the proportion of a compound, you can calculate the mass of one element from the mass of the other, just as you would with a cooking recipe.
Notice that the more hydrogen you add, the more oxygen you need to keep exactly the same $1:8$ proportion.
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In water, hydrogen and oxygen combine in the ratio of $1\ \text{g}$ of hydrogen for every $8\ \text{g}$ of oxygen. How many grams of oxygen will combine with…
In water, hydrogen and oxygen combine in the ratio of $1\ \text{g}$ of hydrogen for every $8\ \text{g}$ of oxygen. How many grams of oxygen will combine with $4\ \text{g}$ of hydrogen?
To form carbon dioxide, carbon and oxygen combine in the ratio of $3\ \text{g}$ of carbon for every $8\ \text{g}$ of oxygen. What mass of oxygen do you need to…
To form carbon dioxide, carbon and oxygen combine in the ratio of $3\ \text{g}$ of carbon for every $8\ \text{g}$ of oxygen. What mass of oxygen do you need to completely burn $9\ \text{g}$ of carbon?
Calcium and oxygen combine in the ratio $5:2$ to form quicklime (calcium oxide). a) What mass of calcium combines with $16\ \text{g}$ of oxygen? b) What mass of…
Calcium and oxygen combine in the ratio $5:2$ to form quicklime (calcium oxide).
a) What mass of calcium combines with $16\ \text{g}$ of oxygen?
Proust's law also works the other way round: if you are given two samples with different masses of the same elements, you can check whether they are consistent with the same compound by calculating the proportion for each one.
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One water sample contains $2\ \text{g}$ of hydrogen and $16\ \text{g}$ of oxygen; another sample contains $5\ \text{g}$ of hydrogen and $40\ \text{g}$ of oxygen. The…
One water sample contains $2\ \text{g}$ of hydrogen and $16\ \text{g}$ of oxygen; another sample contains $5\ \text{g}$ of hydrogen and $40\ \text{g}$ of oxygen. The figure compares the composition of the two samples.
Calculate the ratio between hydrogen and oxygen in each sample and reason whether they could be the same compound.
One sample contains $3\ \text{g}$ of carbon and $8\ \text{g}$ of oxygen, and another contains $3\ \text{g}$ of carbon and $4\ \text{g}$ of oxygen. Could they be the same…
One sample contains $3\ \text{g}$ of carbon and $8\ \text{g}$ of oxygen, and another contains $3\ \text{g}$ of carbon and $4\ \text{g}$ of oxygen. Could they be the same compound? Justify your answer using Proust's law.
A laboratory analyses two samples of an iron oxide. Sample A contains $14\ \text{g}$ of iron and $6\ \text{g}$ of oxygen; sample B contains $21\ \text{g}$ of iron and…
A laboratory analyses two samples of an iron oxide. Sample A contains $14\ \text{g}$ of iron and $6\ \text{g}$ of oxygen; sample B contains $21\ \text{g}$ of iron and $9\ \text{g}$ of oxygen. Calculate the ratio for each and decide if they are the same compound.
Sometimes two elements combine in more than one fixed proportion, and each proportion gives rise to a different compound, with different properties.
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Copper can form two different oxides. In one sample, $8\ \text{g}$ of copper combine with $2\ \text{g}$ of oxygen; in another sample, $8\ \text{g}$ of copper combine…
Copper can form two different oxides. In one sample, $8\ \text{g}$ of copper combine with $2\ \text{g}$ of oxygen; in another sample, $8\ \text{g}$ of copper combine with $1\ \text{g}$ of oxygen. The figure compares the composition of the two samples.
We analyse three samples of "iron oxide". Sample A contains $14\ \text{g}$ of iron and $6\ \text{g}$ of oxygen; sample B, $14\ \text{g}$ of iron and $4\ \text{g}$ of…
We analyse three samples of "iron oxide". Sample A contains $14\ \text{g}$ of iron and $6\ \text{g}$ of oxygen; sample B, $14\ \text{g}$ of iron and $4\ \text{g}$ of oxygen; sample C, $21\ \text{g}$ of iron and $9\ \text{g}$ of oxygen.
a) Calculate the iron:oxygen ratio for each sample.
b) Which of them are the same compound?
c) Explain why iron can give rise to more than one different oxide.
Carbon and oxygen can form two different gases. In sample A, there are $6\ \text{g}$ of carbon and $8\ \text{g}$ of oxygen; in sample B, there are $6\ \text{g}$ of…
Carbon and oxygen can form two different gases. In sample A, there are $6\ \text{g}$ of carbon and $8\ \text{g}$ of oxygen; in sample B, there are $6\ \text{g}$ of carbon and $16\ \text{g}$ of oxygen. The figure shows the model of each molecule.
a) Calculate the carbon:oxygen ratio for each sample.
b) Are they the same compound?
c) Given that one of the gases is carbon monoxide (highly toxic) and the other is carbon dioxide, explain why Proust's law allows us to distinguish between them even though they are made of the same elements.
If you mix two elements without respecting their exact proportion, not everything reacts: the one in excess is left over without combining.
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In a sealed container, we place $10\ \text{g}$ of hydrogen and $64\ \text{g}$ of oxygen and cause them to react (the combination ratio is $1:8$). The figure shows the…
In a sealed container, we place $10\ \text{g}$ of hydrogen and $64\ \text{g}$ of oxygen and cause them to react (the combination ratio is $1:8$). The figure shows the container before the reaction.
We mix $16\ \text{g}$ of hydrogen with $100\ \text{g}$ of oxygen and trigger the reaction (ratio $1:8$). a) What mass of each gas reacts? b) What mass of water is…
We mix $16\ \text{g}$ of hydrogen with $100\ \text{g}$ of oxygen and trigger the reaction (ratio $1:8$).
A student claims: "If I add twice as much oxygen, the water that forms will contain twice as much oxygen." To test this, we combine $4\ \text{g}$ of hydrogen with…
A student claims: "If I add twice as much oxygen, the water that forms will contain twice as much oxygen." To test this, we combine $4\ \text{g}$ of hydrogen with $64\ \text{g}$ of oxygen (twice as much as necessary).
a) Calculate the mass of water formed and the mass of oxygen left over.
b) Use Proust's law to explain why the student is mistaken.