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# Chemistry help? STP! I don't understand this part in Chem!?

This is only for a review paper but I don't want to put nothing down on it. Can you include all your work for the problems so I can see what you did to better understand what we are doing in class. My teacher will go over this stuff again of tuesday if I still need it.

1. Use Boyle’s Law to solve the following problem:

A sample of oxygen gas at 295 kPa takes up 456 L of space at standard temperature (the “T” in STP). What is the new volume when the pressure is lowered to standard pressure (the “P” in STP)?

2. Use Charles’ Law to solve the following problem:

A flexible container holds 2003 cm3 of air at 285 K. What would the new volume be if the container were cooled to 18 C? (REMEMBER: You will also need to use the conversion from degrees C to degrees K!)

3. Use Gay-Lussac’s Law to solve the following problem:

At 195 C, the pressure exerted by oxygen gas is 754 kPa. What will the pressure be at 568 K, assuming constant volume? (REMEMBER: You will also need to use the conversion from degrees C to degrees K!)

### 2 Answers

- Dr WLv 77 years ago
if you start with this equation

.. PV = nRT

and rearrange

.. PV / (nT) = R

and since that R is a constant... ANY PV / (nT) must = any other PV / (nT)

and for 2 gases.... or a gas in 2 different states.

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

******

now if we take the equation

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

and start holding different variables constant...so they cancel out..

..example..

... .if P1 = P2...

... .then P2V1 / (n1T1) = P2V2 / (n2T2)

.... and P2 cancels out leaving

.. ....V1 / (n1T1) = V2 / (n2T2)

*****

anyway.. holding different variables constant gives these 15 equations

zero variables held constant

(1).. [ P1V1 / (n1T1) = P2V2 / (n2T2) ]... nothing constant... not named

1 variable held constant

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

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

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

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

2 variables held constant

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

(7).. [ V1/T1 = V2T2 ]... .. ....P & n held constant... "Charles law"

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

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

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

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

3 variables held constant

(12).. [ T1 = T2 ]... .. .. P & V & n held constant... not named

(13).. [ n1 = n2 ]... .. ...P & V & T held constant... not named

(14).. [ V1 = V2 ]... .. ...P & n & T held constant... not named

(15).. [ P1 = P2 ]... .. ...V & T & n held constant... not named

*** equation (9) is erroneously called "Gay Lussac's law" by some instructors.

.. why do I say it's erroneously called that?

.. . .(a) Gay Lussac studied gases at constant P and moles same as

.. .. .... Charles did. never constant volume !!!

... ..(b) the other gas laws at constant moles were all named, so somewhere along

.... .. ...the line some instructors found the need to name the

.. .. .. . P1/T1 = P2/T2 law... and the only "gas law scientist" without a gas law

... ... ...to his name was Mr Lussac. So they called it Gay Lussac's law in err

. . . (c) the scientist who actually studied gases at constant volume was

. ... .. . Guillaume Amonton.

. ... .. ..see the link herehttp://www.lookchem.com/Chempedia/Basic-Chemical/C...

.. . .(d) we called that "John Doe's law" back when I took general chem.

******

regardless...

.. (1) all 15 of those gas laws are different variations of the ideal gas law with

... ... different variables held constant.

.. (2) some are named and some are not

.. (3) there is a better way of solving gas law problems then memorizing

... .. .those 15 variations, their names, for which conditions they apply

.. .. .and praying to God that you choose the right equation come

.. ....exam time.

******

the better way is this

.. (1) start with the equation P1V1 / (n1T1) = P2V2 / (n2T2)

.. (2) rearrange for your desired unknown

.. (3) identify and cancel anything held constant

.. (4) plug in the data and chug out the answer

IF you MUST remember the names... because of a foolish instructor...

.. (1) "combined gas law"... only "n" constant

.. (2) "Charles law"... .. n + P constant..

.. (3) "boyles law".. .. .. n + T constant

.. (4) "John Doe's law" .n + V constant.... this is your "Gay Lussac law"

.. (5) "Avogadro's law"..P + T constant... this one is different from the rest!

combined is easy right? just n

boyles + Charles + john doe's law are easy.. n + something else

.. for boyles... think boiling = temperature.. n + T

.. for Charles law... you're going to need to remember pressure

.. that leaves john doe

Avogadro's is different... you'll have to remember P&T are constant

******

so... all that said.. this is how to solve those 3 problems using that method I outlined above

*** 1 ***

starting with

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

rearranging for V2... that's what the problem asked for

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

assuming n and T are constant since they are not mention and we have another hint.. the problem states use Boyles law (that's the one with n and T held constant)

.. V2 = V1 x (P1/P2)

solving

.. V2 = 456L x (295kPa / 101.325 kPa) = __ L

note that I'm assuming STP here means the usual

.. P = 1atm = 101.325 kPa

.. T = 273.15K

*** 2 ***

exactly the same way !!!!

starting with

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

rearranging for V2... that's what the problem asked for

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

assuming n and P are constant since they are not mentioned and the problem states to use Charles law (n and P held constant)

.. V2 = V1 x (T2/T1)

solving

.. V2 = 2003 cm³ x (18+273.15 K / 285K) = __cm³

double check that 18°C. it's written strangely in your problem statement

*** 3 ***

same thing.. but I'm going to skip the comments

P1V1 / (n1T1) = P2V2 / (n2T2)

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

P2 = P1 x (T2 / T1)

P2 = 754 kPa x (568K / (195 + 273.15 K)) = ___ kPa

*** notes ***

(1) STP may mean

.. P = 1atm = 100 kPa

.. T = 273.15K

... make sure you ASK YOUR INSTRUCTOR which convention he or she is using

(2) you get to do the math

(3) notice I didn't have to sort through those equations in my

... .head to pick the correct one to use? The correct ones

.. .came about naturally by using my method. Can you find

... .. .. P1/V1 = P2/V2 ===> .. V2 = V1 x (P1/P2)

... and the rest in my solutions?

**********

how much of this did you follow?

- donpatLv 77 years ago
Hello Catlin : STP referts to " Standard Temperature and Standard Pressure " :

-----------------------------------------------------------------------------------------------------------------------

For the metric or SI System, you normally have for STP :

-------------------------------------------------------------------------------------------

STP T = 0.0 C = 273.2 K

STP P = 1.0 atm = 101.325 kPa = 101325 Pa = 760 mm Htg = 760 torr

According to Boyle's Law :

----------------------------------------

PV = Constant for ideal gas when the temperature is constant

( P1 ) ( V1 ) = ( P2 ) ( V2 )

V2 = ( V1 ) ( P1 / P2 )

V2 = ( 456 L ) ( 295 kPa / 101.325 kPa ) = 1328 L <----[ 1 ]-------

According to Charles" Law :

---------------------------------------------

V / T = Constant for ideal gas at constant pressure

( V1 ) / ( T1 ) = ( V2 ) / ( T2 )

V2 = ( V1 ) ( T2 / T1 )

V2 = ( 2003 cu cm ) ( 291.2 K / 285 K ) = 2047 cu cm <------[ 2 ]-------

According to Gay-Lussac's Law :

---------------------------------------------------------

P / T = Constant fot ideal gas at a constant volume :

P1 / T1 = P2 / T2

P2 = ( P1 ) ( T2 / T1 )

P2 = ( 754 kPa ) ( 568 K / 468.2 ) = 915b kPa <-------------

All this is can be much simpler by using the following equation for a fixed moles

or mass of and ideal gas :

----------------------------------------------

( P2 ) ( V2 ) / ( T2 ) = ( P1 ) ( V1 ) / ( T1 )

This equation comes from the Ideal Gas Law :

------------------------------------------------------------------

PV = nRT

PV / T = nR = Constant

( P2 ) ( V2 ) / ( T2 ) = ( P1 ) ( V1 ) / ( T1 ) <----------------------

The above equation requires the following :

( 1 ) Ideal gas

( 2 ) Fixed moles or mass of the gas

( 3 ) Use of absolute temperatures

( 4 ) Use of absolute pressures

The above equation may be expanded to be usable for moles of gas varying :

PV = nRT

PV / nT = R

( P2 ) ( V2 ) / ( n2 ) ( T2 ) = ( P1 ) ( V1 ) / ( n1 ) ( T1 ) <------------------

This equation requires :

( 1 ) Ideal gas

( 2 ) Use of absolute temperatures

( 3 ) Use of absolute pressures

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