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# Thread: Chemistry Help Needed(Reps for help)

1. ## Chemistry Help Needed(Reps for help)

I don't need the answers, because i already have them, but i need to know how to do the following questions:

1. If the pressure of a gas sample is quadrupled and the ablsolute temperature is doubled, by what factr does the volume of the sample change?

2. At what temp will a fixed amount of gas with a volume of 175 L at 15 degree celcius and 760 mmHg occupy a volume of 198 L at a pressure of 640 mm Hg?

3. Calculate the volume occupied by 25.2 g of CO2 at 0.84 atm and 25 degrees celcius.

4.How many molecules of N2 gas can be present in a 2.5L flask at 50 degree celcius and 650 mmHg?

5. Calculate the density in g/L of CO2 gas at 27 degree C and 0.5 atm pressure.

6. 1.018 g of Freon -113 gas is trapped in a 145 mL container at 760 mm Hg and 50.0 degree C what is the molar mass of freon 113?

I know, these questions may be easy for you, but i have NO idea how to do them since my teacher doesnt really teacha nd just expects us to learn from the book (which is hard because the explanations suck). Thank you.

2. pv=nrt

3. they are easy but just put in the equation...i dont wanna go look for it now

4. 1) Green

2) Pencil

3) Hedgehog

4) Not sure on this one

5) 139

6) Scott Bacula

hope this helps

5. Originally Posted by colt.45
pv=nrt
yea thats it, temperature needs to be in Calvin, n = moles and r is constant depending on the unit you used for pressure

6. Thanks, i guess...

7. ideal gas law: (pressure: atm)(volume: liter)=(mole)(gas constant: .08206 L atm/ mole Kelvin)(temperature: Kelvin)

PV=nRT

8. Originally Posted by KenshinR1
ideal gas law: (pressure: atm)(volume: liter)=(mole)(gas constant: .08206 L atm/ mole Kelvin)(temperature: Kelvin)

PV=nRT
I know that, but i did it for number 3 and i came out with 733.974 L and the answer is 16.9 L. I did something wrong. this is what i did:

0.84V=(25.2)(0.0821)(298)

Thats PV=nRT With everything substituted exept for V because thtas what im trying to find. I divided the left and right side of the equation by 0.84 and V=734
but the answer is 16.9. I dont get it.

9. 1. half

I hope you know the units for volume, temperature, pressure, moles, and pressure.
Once you figure those out - use the PerVert = NeRT equation listed above.

10. Originally Posted by Lol312
I don't need the answers, because i already have them, but i need to know how to do the following questions:

1. If the pressure of a gas sample is quadrupled and the ablsolute temperature is doubled, by what factr does the volume of the sample change?

Decreases by 2 ....v=nrt/p = V = nr*2/4

2. At what temp will a fixed amount of gas with a volume of 175 L at 15 degree celcius and 760 mmHg occupy a volume of 198 L at a pressure of 640 mm Hg?

pv=nrt....I guess calculate n and plug it into new equation using BOLDED numbers

3. Calculate the volume occupied by 25.2 g of CO2 at 0.84 atm and 25 degrees celcius.

convert 25.3g to moles (12.01 +32 g/mol)...use that to obtain V

4.How many molecules of N2 gas can be present in a 2.5L flask at 50 degree celcius and 650 mmHg?

use pv=nrt to calculate n (moles) and then use Avocados number to obtain # of molecules

5. Calculate the density in g/L of CO2 gas at 27 degree C and 0.5 atm pressure.

6. 1.018 g of Freon -113 gas is trapped in a 145 mL container at 760 mm Hg and 50.0 degree C what is the molar mass of freon 113?

I know, these questions may be easy for you, but i have NO idea how to do them since my teacher doesnt really teacha nd just expects us to learn from the book (which is hard because the explanations suck). Thank you.
I will read 5 and 6 in 5 min and answer them

pv=nrt

Pressure - p
Volume = v
mols = n
R is constant = 0.083 etc
Temp = t

11. Originally Posted by Lol312
I know that, but i did it for number 3 and i came out with 733.974 L and the answer is 16.9 L. I did something wrong. this is what i did:

0.84V=(25.2)(0.0821)(298)

Thats PV=nRT With everything substituted exept for V because thtas what im trying to find. I divided the left and right side of the equation by 0.84 and V=734
but the answer is 16.9. I dont get it.
that is 25.2 grams, you gotta convert it to moles

12. Originally Posted by KenshinR1
that is 25.2 grams, you gotta convert it to moles
Oh, ok let me try that now. *reps to both above posters*

13. Whoah, thanks Kenshin, i got the right answer.

14. Originally Posted by colt.45
pv=nrt
dude this is all you need to know!!

15. Originally Posted by Lol312
I don't need the answers, because i already have them, but i need to know how to do the following questions:

1. If the pressure of a gas sample is quadrupled and the ablsolute temperature is doubled, by what factr does the volume of the sample change?

2. At what temp will a fixed amount of gas with a volume of 175 L at 15 degree celcius and 760 mmHg occupy a volume of 198 L at a pressure of 640 mm Hg?

3. Calculate the volume occupied by 25.2 g of CO2 at 0.84 atm and 25 degrees celcius.

4.How many molecules of N2 gas can be present in a 2.5L flask at 50 degree celcius and 650 mmHg?

5. Calculate the density in g/L of CO2 gas at 27 degree C and 0.5 atm pressure.

6. 1.018 g of Freon -113 gas is trapped in a 145 mL container at 760 mm Hg and 50.0 degree C what is the molar mass of freon 113?

I know, these questions may be easy for you, but i have NO idea how to do them since my teacher doesnt really teacha nd just expects us to learn from the book (which is hard because the explanations suck). Thank you.
5. pv=nrt becomes n/v = p / rt where I believe u will get mol/L and use CO2 (44.01 g/mol) to obtain g/L

6. pv=nrt just find n (moles) and divide 1.018 g by moles

I assume u will convert all of these to right units

16. #4 find the mols of gas and multiply it by avogadro's number 6.0221415 ? 10^23

17. Originally Posted by TunaMan5
5. pv=nrt becomes n/v = p / rt where I believe u will get mol/L and use CO2 (44.01 g/mol) to obtain g/L

6. pv=nrt just find n (moles) and divide 1.018 g by moles

I assume u will convert all of these to right units
can you teach me number 5 step by step please? i dont understand it at all.

19. 5. Calculate the density in g/L of CO2 gas at 27 degree C and 0.5 atm pressure.

P=0.5 atm
T=300 K
R= 8.314 L*kPa/(K*mol)

convert atm to kPa. 1 atm=101.3 kPa; therefore 0.5 atm=50.65 kPa

solve for n/V.

n/V=P/RT
n/V=50.65/8.314/300 =0.020307 mol/L

your molecular weight is 16+16+12=44 g/mol

density = MW * n/V = 44 g/mol * 0.020307 mol/L = 0.8935 g/L

20. Originally Posted by Lol312
5. Calculate the density in g/L of CO2 gas at 27 degree C and 0.5 atm pressure.
PV=nRT

density = mass/volume

so (n/V) = P/RT

(.5 atm / .08206 L atm * 300K ) x 44 g/mol
mol K

21. Originally Posted by Lol312
I don't need the answers, because i already have them, but i need to know how to do the following questions:

1. If the pressure of a gas sample is quadrupled and the ablsolute temperature is doubled, by what factr does the volume of the sample change?

2. At what temp will a fixed amount of gas with a volume of 175 L at 15 degree celcius and 760 mmHg occupy a volume of 198 L at a pressure of 640 mm Hg?

3. Calculate the volume occupied by 25.2 g of CO2 at 0.84 atm and 25 degrees celcius.

4.How many molecules of N2 gas can be present in a 2.5L flask at 50 degree celcius and 650 mmHg?

5. Calculate the density in g/L of CO2 gas at 27 degree C and 0.5 atm pressure.

6. 1.018 g of Freon -113 gas is trapped in a 145 mL container at 760 mm Hg and 50.0 degree C what is the molar mass of freon 113?

I know, these questions may be easy for you, but i have NO idea how to do them since my teacher doesnt really teacha nd just expects us to learn from the book (which is hard because the explanations suck). Thank you.
OK .....You know that the equation is pv=nrt
u are given Temp and Pressure, R is constant so it is always the same

now u rearrange the formula and u will get n (mol) /v (L) = p (atm) / r (.08206 L atm/ mole Kelvin) t (Kelvin) where u will get a result in mol / L because atm/Kelvin cancel ....

Now u have n / Volume = mol / L and because u want the answer in g/L u use g/mol of CO2 which is 44.01 g/mol

SO.... n/V = mol/L * 44.01 g/mol = g/L

There

22. so u get it now...

23. thank you.

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