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# Gas Laws HELP!?

i will be having a quiz on Boyles Law, Charles Law, Gay-Lussac's Law, and the Combined Gas Law.

On my quiz we are going to be given 4 different questions which need to be solved using one of the laws. How do I know which question uses what law?

Please answer ASAP

Thanks

### 3 Answers

- Dr WLv 75 years agoFavorite Answer
if you start with the ideal gas law

.. PV = nRT

rearrange

.. PV / (nT) = R

and recognize that R is a constant.. so that all PV/(nT)'s must therefore be equal

.. P1V1 / (n1T1) = P2V2 / (n2T2)

now.. if we start holding different things constant.. let's say we hold moles constant so that n1 = n2, things start to drop out of that equation. And we can generate these permutations

******

no variables held constant

.. (1).. [ P1V1 / (n1T1) = P2V2 / (n2T2) ]... . not named

1 variable held constant

.. (2).. [ V1 / (n1T1) = V2 / (n2T2) ].. .. P constant... . not named

.. (3).. [ P1 / (n1T1) = P2 / (n2T2) ].. .. V constant.. .. not named

.. (4).. [ P1V1 / T1 = P2V2 / T2 ].. .. ....n constant.. .. ."combined gas law"

.. (5).. [ P1V1 / n1 = P2V2 / n2 ].. .. .. .T constant.. . ..not named

2 variables constant

.. (6).. [ n1T1 = n2T2 ].. . .. .. ... ...P & V constant.. . .not named

.. (7).. [ V1 / T1 = V2 / T2 ].. ... .. ..P & n constant.. . .."Charles law"

.. (8).. [ V1 / n1 = V2 / n2 ].. .. .. .. P & T constant.. .. ."Avogadro's law"

.. (9).. [ P1 / T1 = P2 / T2 ].. . . .....V & n constant.. .. "Amonton's law"

.(10).. [ P1 / n1 = P2 / n2 ].. .. .. ....V & T constant.. . .not named

.(11).. [ P1V1 = P2V2 ].. .. .... .. .....n & T constant.. .. "Boyles law"

and there are a few trivial ones with 3 and 4 variables held constant that are not named either.

so.. if you look at those 11 equations, you can readily spot

.. (a).. most of the named ones all have moles and maybe something else constant

.. (b).. #(9) is "Amonton's law".. not "Gay Lussac's law".. I don't know where that

.. .. . ..Gay Lussac nonsense started. But.. it's time I do my part to put and end to it

.. .. ... Guillaume Amonton discovered this relationship 100 years before Joseph Louis

.. .. ... Gay-Lussac came along.. AND... Mr Gay-Lussac never worked on combining

.. ... .. gases at constant volume & moles. Show this to your instructor and kindly ask

.. .. .. .him or her to google this topic and stop propagating this erroneous info.

*********

anyway... the real issue here is

.. (a) do you really want to memorize all 11 of those equations, for what conditions they

.. .. ...apply and their names?

.. (b) hope to God that you remember those equations, for what conditions they apply

.. .. ..and get them jotted down right come test time... or.. project time in your career?

because there is a better way for handling these two state gas law type problems

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the better method

.. (1) write down P1V1 / (n1T1) = P2V2 / (n2T2)... which you'll remember from the ideal gas law

.. (2) rearranging for your desired unknown

.. (3) identify and cancel anything held constant

.. (4) plug in the data and chug out the numbers... remember T in K or R never °C nor °F

*******

*******

examples

a rigid sealed container was cooled from T = 350K to T = 325K. The original pressure was 2.5atm, what's the new pressure?

.. P1V1 / (n1T1) = P2V2 / (n2T2)

.. P2 = P1 x (V1 / V2) x (n2 / n1) x (T2 / T1)

.. P2 = P1 x (T2 / T1)... . <=== rigid means V is constant... sealed means is constant

.. P2 = 2.5atm x (300K / 325K) = 2.3atm

easy right? and you'll notice that P2 = P1 x (V1 / V2) is BOYLES LAW. see how that came about naturally without me fussing over all those named laws and when they apply?

another example

.. A rigid container was cooled from 400K to 350K. By what factor do moles of gas need to be increased to increase the pressure by 10% over it's original value?

.. P1V1 / (n1T1) = P2V2 / (n2T2)

.. n2 = n1 x (P2 / P1) x (V2 / V1) x (T1 / T2)

.. n2 = n1 x (P2 / P1) x (T1 / T2)... .. .<=== V is constant

.. n2 = n1 x (1.1P1 / P1) x (400K / 350K) = 1.2 x n1 = 20% increase

and notice this equation

.. n2 = n1 x (P2 / P1) x (T1 / T2)

is really

.. P1 / (n1T1) = P2 / (n2T2).... . #3 on the list I gave you.. .and it's not named!

if you just memorize those named equations, you might not know how to solve all the problems!.. .and you might get the equations mixed up along the way.

*******

if your instructor really really wants you to memorize equations 4, 7, 8, 9, and 11 above.. well you could try this..

.. combined.... . "n".. only is constant

.. Boyles... .. . ..."n & T"... boyles.. boiling.. temperature.. get it?

.. Charles..... ...."n & P".. constant... pressure.. Charles....

.. Amontons.. .. "n & V"... the V is like an upside down A... and forget Gay Lussac

.. and of course Avogadro's law.. V1/n1 = V2/n2

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- pisgahchemistLv 75 years ago
There is no "Gay-Lussacs law" if you are referring to ....

P/T = k

P1/T1 = P2/T2

Gay-Lussac discovered Charles's law, the relationship between volume and temperature at constant pressure. J. Charles never published his findings, but Gay-Lussac did and gave credit to Charles. The relationship between P and T is correctly called Amontons's law.

There is a "Gay-Lussac's law of combining volumes". For instance, at the same temperature and pressure two volumes of hydrogen will react with one volume of oxygen to make two volumes of water vapor.

2H2(g) + O2(g) --> 2H2O(g)

Boyle's law .... deals with P and V at constant T and n

Charles's law .... deals with V and T at constant P and n

Amontons's law .... deals with P and T at constant V and n

Avogadro's hypothesis .... deals with V and n at constant P and T

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1. Unless you are faced with a question about identifying Boyle's law or Charles's law, etc, you can simply use the combined gas law for any problem involving changes in pressure, volume or temperature of a confined gas (where the moles are constant), and then simply cancel out any variable which is constant.

P1V1 / T1 = P2V2 / T2

2. For any problem which involves the number of moles, or something related like the mass, density or molar mass, use the ideal gas equation.

PV = nRT

Substitute the following as needed

n = m / M .... where n is moles, m is mass, M is molar mass

...... PV = mRT / M

D = m / V .... where D is density, m is mass, V is volume

...... P = m/V x RT/M ..... P = DRT/M

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- 5 years ago
If only pressure and volume are mentioned in the question and the temperature is constant, be sure to use Boyle's law.

in case of gay lussac's law or Charles law... pressure will be constant.

In combined law... we will see that v=RTn/P where n will be the no of moles

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