[Jennifer Anderson]

Hi everyone.  :)I'm really having trouble with this section and was hoping someone could show me how to solve one of these problems. 

The O2 gas produced in the decomposition of sodium chlorate was collected into a 475-mL container over water at a temperature of 16 degrees celcius. At this temperature, the vapor pressure of water is 14mm Hg. If the total pressure in the container was 730 mm Hg, calculate the mass of NaCl that the reaction produced.

thank you 

[Borek]

Anyone?

Please read forum rules.

You have to show your attempts at solving the question to receive help. This is a forum policy.

Start with the reaction equation. What was partial pressure of oxygen? Moles of oxygen?

[Jennifer Anderson]

Hi, sorry about that.

heres the balanced reaction equation that i got

NaClO3 -->  NaCl + 3/2 O2                   

and as far as i know the partial pressure of O2 is 160

can you show me how to set up the rest of the problem?

[Borek]

Partial pressure of oxygen in the container is not 160 mm Hg.

What two gases are present in the container?

[Jennifer Anderson]

oh sorry! so we have O2 and H20 (water vapor) in the container

i know that the total pressure is the sum of the partial pressures but i don't know how to find them.

isn't the partial pressure of H20 given as 14mm hG? how do i find the P of o2?


once i get the values figured out don't i just plug them into PV = nRT and solve for n?

thanks for being patient i'm really bad at this

[Borek]

i know that the total pressure is the sum of the partial pressures but i don't know how to find them.

isn't the partial pressure of H20 given as 14mm hG? how do i find the P of o2?

You have properly listed almost all information needed to calculate partial pressure of oxygen. What is the total pressure in the container?

once i get the values figured out don't i just plug them into PV = nRT and solve for n?

Yes.

[Jennifer Anderson]

730 mm Hg - 14 mm Hg = 716 mm Hg (partial pressure of O2)

arranging the formula i got n = PV/RT

so

(716 mm Hg)(0.475 L)
--------------------- =   1197185.14
    (0.0821)(289 K)


that doesn't seem right what am i doing wrong?

[Borek]

Check your units and units of the ideal gas constant you used.

[Jennifer Anderson]

hmm...i'm not entirely sure what you mean... like this?


(716 mm Hg)(0.475 L)
--------------------- =          .09  g NaCl
(62.4 mm Hg per mol)(289 K)

[Jennifer Anderson]

no wait wouldn't that be mol NaCl which i then convert to grams?

[Borek]

Take a look at the table on the right:

http://en.wikipedia.org/wiki/Ideal_gas_constant

You were doing OK, but the value you have used for R was the one fit for pressure in atm, while you have pressure in mmHg, hence your result was off.

[Jennifer Anderson]

62.4 is the value for mmHg, did you see the updated version i did?

(716 mm Hg)(0.475 L)
--------------------- =          .09  mol/L  NaCl
(62.4 mm Hg per mol . K)(289 K)


i know i would need that in three significant figures, i just wanted you to check an see if that is the correct formula before i go ahead.
i'm worried because the question wants the mass of NaCl produced and it looks like i am left with the molarity

[Borek]

(716 mm Hg)(0.475 L)
--------------------- =          .09  mol/L  NaCl
(62.4 mm Hg per mol . K)(289 K)

Units, units, units. 62.4 - value is OK, but units are not, you ate L. Even then using units you have listed it should be mol*L, not mol/L.

Besides, you are not calculating number of moles of NaCl, but number of moles of gas (do you still remember which one?)