# Exploring Avogadro's Law: Equal Molecules, Equal Volumes?

• gracy
In summary: In this case, the mass of water will be the same in both liquid and vapor phases since the water vapor will eventually condense into liquid form. However, for calculation purposes, it is easier to consider the water as a gas in the initial stages of the reaction. In summary, Avogadro's Law states that equal volumes of gases at the same temperature and pressure contain the same number of molecules or moles of gas. This law can be applied in both directions, meaning that gases of equal moles at the same temperature and pressure will have equal volumes. This was originally observed through Gay-Lussac's Law. The mass of water remains the same in both vapor and liquid phases, but for calculation purposes, it is often easier to consider
gracy
Avogadro's Law states that 'equal volumes of gases at the same temperature and pressure contain the same number of molecules' or moles of gas.
My question is ,will it be correct if we reverse the law,
I mean if gases of equal moles at the same temperature and pressure will have equal volumes,is it right?For eg.
• N2(g) + 3H2(g) ==> 2NH3(g)
• 1 mole of nitrogen gas combines with 3 moles of hydrogen gas to form 2 moles of a ammonia gas.we can say that 1 volume (what ever be the unit) of nitrogen reacts with 3 volumes of hydrogen to produce 2 volumes of ammonia?
• provided that the gases are at same temperature and pressure.

Yes, it works both ways.

(As long as the ideal approximation holds)

gracy
Borek said:
As long as the ideal approximation holds)
• provided that the gases are at same temperature and pressure.

No. PV=nRT is an "ideal gas approximation" - that is, it works only for ideal gases. No gas is ideal, but as long as the distances between molecules are large enough, every gas can be approximated as ideal.

http://en.wikipedia.org/wiki/Ideal_gas (just read the initial part).

gracy
I have one more problem
• Given the equation: 2H2(g) + O2(g) ==> 2H2O(l)
If 40 dm3 of hydrogen, (at 25oC and 1 atm pressure) were burned completely ...What mass of water is formed?
• In my textbook solution is as follows
• The easiest way to solve this problem is to think of the water as being formed as a gas-vapour.
• The theoretical gas volume ratio of reactant hydrogen to product water is 1 : 1
• Therefore, prior to condensation at room temperature and pressure, 40 dm3 of water vapour is formed.
• 1 mole of gas occupies 24 dm3, and the relative molar mass of water is 18 g/mol
• (atomic masses H = 1, O = 16, so Mr(H2O) = 1 + 1 + 16 = 18).
• Therefore moles of water formed = 40/24 = 1.666 moles
• Since moles = mass / formula mass
• mass = moles x formula mass
• mass water formed = 1.666 x 18 = 30g of H2O
My question is how can we think water in liquid as water in vapor phase?How is it correct to do?

It is a shortcut - if water were a gas, vapor would occupy 40 L. Yes, it condenses next, but assuming it is still a gas after the reaction simplifies calculations. You can think in terms of "when gases react, product is gaseous, and the condensation is not instantaneous, so the product is gaseous for long enough we can calculate its amount" ;)

gracy said:
Avogadro's Law states that 'equal volumes of gases at the same temperature and pressure contain the same number of molecules' or moles of gas.
My question is ,will it be correct if we reverse the law,
I mean if gases of equal moles at the same temperature and pressure will have equal volumes,is it right?For eg.
• N2(g) + 3H2(g) ==> 2NH3(g)
• 1 mole of nitrogen gas combines with 3 moles of hydrogen gas to form 2 moles of a ammonia gas.we can say that 1 volume (what ever be the unit) of nitrogen reacts with 3 volumes of hydrogen to produce 2 volumes of ammonia?
• provided that the gases are at same temperature and pressure.

Yes, in fact empirical observations of just such combining volumes, subsumed in 'Gay-Lussac's Law' were most of the motivation of the Avogadro hypothesis as it was originally.

I wanted to ask will mass of water be same in both vapor and liquid phase?If yes,then we can calculate mass of water in any of these phases depending upon ease.

gracy said:
I wanted to ask will mass of water be same in both vapor and liquid phase?

Yes, mass is conserved.

## 1. What is Avogadro's Law?

Avogadro's Law states that equal volumes of gases, at the same temperature and pressure, contain the same number of molecules. This means that for any gas, the volume of the gas is directly proportional to the number of molecules present.

## 2. Who discovered Avogadro's Law?

The law is named after Italian scientist Amedeo Avogadro, who proposed it in 1811 as a way to explain the behavior of gases.

## 3. How does Avogadro's Law relate to the Ideal Gas Law?

The Ideal Gas Law combines Avogadro's Law with three other gas laws (Boyle's Law, Charles's Law, and Gay-Lussac's Law) to create a formula that describes the behavior of gases under various conditions. It states that the pressure, volume, and temperature of a gas are all related and can be calculated using the equation PV = nRT, where n is the number of moles of gas, R is the gas constant, and T is the temperature in Kelvin.

## 4. What are some real-life applications of Avogadro's Law?

Avogadro's Law is used in various industries, such as in the production of chemicals and fuels. It is also used in the production of food and beverages, as well as in the field of medicine for measuring the amount of gas in a patient's body. Additionally, it is a fundamental concept in chemistry and is used to explain the behavior of gases in chemical reactions.

## 5. Are there any exceptions to Avogadro's Law?

While Avogadro's Law holds true for most gases, there are some exceptions. For instance, at very high pressures and low temperatures, the volume of gases can deviate from what is predicted by the law. Additionally, gases that undergo reactions or have strong intermolecular forces may not follow the law accurately.

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