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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: xela89 on May 21, 2006, 08:29:10 PM

Title: Various General Chemistry Questions
Post by: xela89 on May 21, 2006, 08:29:10 PM
Edited to show attempts: First post w00t.  Just a quick explanation of how to do these problems would be appreciated.  I have gmail invites for those who want them.

1. How much heat (in KiloJoules) is released when 50.0 grams of steam at 100 C is condensed to water at 100 C?
Phase change problem. I don't know where to start although I vaguely remember it utilized a picture of an upsloping line.


2. How many mL of 10 M NaOH are needed to make 500.0 mL of of 1.0 M NaOH?
I don't even know where to start. Molarity=moles solute X Liters solution


3. If 9.5 KJ of energy is absorbed by a block of ice. how many moles of ice would be melted?

4.18KJ = 9.5KJ/moles X 273 degrees K   ???

4.How many grams of NaCl would be needed to make 3.5 L of a 2.0 M solution?
3.5L NaCl X 22.4 L. I'm drawing a blank.

Thanks again.

Title: Re: Various Chemistry Questions - *delete me* - Reward offered!
Post by: mike on May 21, 2006, 08:31:51 PM
Show us your attempts first ;)
Title: Re: Various Chemistry Questions - *delete me* - Reward offered!
Post by: Will on May 21, 2006, 08:45:50 PM
Quote from: xela89 on May 21, 2006, 08:29:10 PM
I have gmail invites for those who want them.

I have 200 invites, so no need to offer a reward- if any established chemist (!) wants a gmail account they can pm/im me!

Like mike said, you need to show you attempted the questions, or what specifically about the question you don't understand.

Surely you can work out how to do a question as simple as question 2! Work out moles of NaOH required, and then divide by the conc. of NaOH you have...
Title: Re: Various Chemistry Questions - *delete me* - Reward offered!
Post by: mike on May 21, 2006, 08:54:18 PM
xela89:

Well question 2 is a dilution problem so you could use the equation:

C1V1 = C2V2

where C - concentration and V - volume
Title: Re: Various Chemistry Questions - *delete me* - Reward offered!
Post by: xela89 on May 21, 2006, 08:55:49 PM
I've never seen that equation before, but it makes sense. Cool.
Title: Re: Various Chemistry Questions - *delete me* - Reward offered!
Post by: Donaldson Tan on May 21, 2006, 08:59:00 PM
Quote from: xela89 on May 21, 2006, 08:29:10 PM
1. How much heat (in KiloJoules) is released when 50.0 grams of steam at 100 C is condensed to water at 100 C?
hint: What is the heat of vapourisation per gram of water?

Quote from: xela89 on May 21, 2006, 08:29:10 PM
2. How many mL of 10 M NaOH are needed to make 500.0 mL of of 1.0 M NaOH?
I don't even know where to start. Molarity=moles solute X Liters solution
hint: Conservation of Mass. The amount of NaOH in the diluted solution is the same as that of the original solution.

Quote from: xela89 on May 21, 2006, 08:29:10 PM
3. If 9.5 KJ of energy is absorbed by a block of ice. how many moles of ice would be melted?
hint: Assume the ice is at 0oC. What is the heat of fusion per kg of ice?

Quote from: xela89 on May 21, 2006, 08:29:10 PM
4.How many grams of NaCl would be needed to make 3.5 L of a 2.0 M solution?
3.5L NaCl X 22.4 L. I'm drawing a blank.
hint: Same hint as question 2.
Title: Re: Various Chemistry Questions - *delete me* - Reward offered!
Post by: mike on May 21, 2006, 09:09:04 PM
Quote
I've never seen that equation before, but it makes sense. Cool.

It comes from the relationship:

n = c.v

n = number of moles

So it just means that if the same number of moles is in solution:

n1 = c1 x v1
and
n2 = c2 x v2

and n1 = n2

then

c1v1 = c2v2
Title: Re: Various Chemistry Questions - *delete me* - Reward offered!
Post by: mrdeadman on May 21, 2006, 11:09:41 PM
Quote from: xela89 on May 21, 2006, 08:55:49 PM
I've never seen that equation before, but it makes sense. Cool.
it's kind of like P1V1=P2V2 pressure is essentially like concentration. same with the equilibrium calculations.