Gen Chem questions

[Golfguy]

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Hi Guys,

I'm having trouble with a couple of problems:

At 0C a gas is dissolved in H20 at a mole fraction of 2 x 10-18. What is the molality of this solution given the molecular weigh of the gas is 32 g/mole?

Answer is 1 x 10-16

and

At 37C, the Kw for H20 is 5 x 10-14. What is the pH of water at this temperature?

I derived the answer from x2 = 5 x 10 -14, to be ~6.6. But why doesn't the [OH-] at the same concentration neutralize this affect of decreased pH?

 

[rockclock]

Hi Guys,

I'm having trouble with a couple of problems:

At 0C a gas is dissolved in H20 at a mole fraction of 2 x 10-18. What is the molality of this solution given the molecular weigh of the gas is 32 g/mole?

Answer is 1 x 10-16

and

At 37C, the Kw for H20 is 5 x 10-14. What is the pH of water at this temperature?

I derived the answer from x2 = 5 x 10 -14, to be ~6.6. But why doesn't the [OH-] at the same concentration neutralize this affect of decreased pH?

#1:

2e-18 mol gas in [1 - 2e-18 = ~1] mol H20...
numerator is mol solute, so you're already good there
denominator is kg solvent, so convert 1 mol H20 into kg = 0.018 kg
2e-18 mol / 0.018 kg = 1e-16 m

#2:

you could do the same calculation and get pOH = 6.6...right? the solution should still be "neutral" since [H+] = [OH-] but if you change Kw, the whole 14 point pH/pOH scale goes out the window...what's the answer given? by pH = log H+, both pH and pOH are "neutral" at 6.6 and now you're operating on a 13.2 scale.

 

[Golfguy]

#1:

2e-18 mol gas in [1 - 2e-18 = ~1] mol H20...
numerator is mol solute, so you're already good there
denominator is kg solvent, so convert 1 mol H20 into kg = 0.018 kg
2e-18 mol / 0.018 kg = 1e-16 m

#2:

you could do the same calculation and get pOH = 6.6...right? the solution should still be "neutral" since [H+] = [OH-] but if you change Kw, the whole 14 point pH/pOH scale goes out the window...what's the answer given? by pH = log H+, both pH and pOH are "neutral" at 6.6 and now you're operating on a 13.2 scale.

Wow, thanks for the great explanations, I completely understand now!

Cold Clock the DAT, by rockclock....in stores soon.

 

[needzmoar]

Wow, thanks for the great explanations, I completely understand now!

Cold Clock the DAT, by rockclock....in stores soon.

 

[rockclock]

Wow, thanks for the great explanations, I completely understand now!

Cold Clock the DAT, by rockclock....in stores soon.

haha no problem. I actually thought about putting something together a while back (at least for QR/GC/OC...the sections you can completely master) but I figure it would make me a total of about $6 after it starts getting pirated haha.

 

[mh0000]

haha no problem. I actually thought about putting something together a while back (at least for QR/GC/OC...the sections you can completely master) but I figure it would make me a total of about $6 after it starts getting pirated haha.

lol. what are you talking about, we would never pirate it... *evil grin while makes scans rockclock's work into pdf*