chemistry
posted by cateye .
you dissolve 6.55g of potassium nitrate in enough water to make a 250ml solution. if you add 1.25 l of additional water to this solution.what is the molarity of this new solution.

mols KNO3 = 6.55g/molar mass KNO3 = ?
M = mols/0.250L = ?M (old M)
New M = old M x [250 mL/(250+1250)] = new M IF (and only if) the volumes are additive; i.e., if 250 mL + 1250 mL actually makes EXACTLY 1,500 mL. 
You have a 4.0 L steel tank containing hydrogen gas. You are able to heat or cool the tank and meaurse the pressure of the gas.
1.If the temperature is 25degree and the pressure is 3.5 aim ..How many grams of hydrogen does the tank contain.
2.at what temperatue in Celsius would you need a pressure of 5.5 atm? 
Use PV = nRT and n = grams/molar mass.

I AM STILL CONFUSE..FOR THE FIRST I CAME OUT WITH .40 BUT FOR NUMBER 2 I STILL CAN FIGURE OUT THAT ONE..PLEASE HELP

I don't thik 0.40 is correct for #1. For #2, I'm not sure what it is asking?
#1.
n = PV/RT = 3.5*4/0.08206*298, then
grams = mols x molar mass. I get something like 1.1 or so. You need to do it a little more accuartely than that.
#2. Could be asking for "What T in celsius is needed to obtain a pressure of 5.5 atm?"
T(in kelvin) = PV/nR = 5.5*4/0.08206*0.572
Then 273+C = K and solve for C. 
unbalanced chemical reaction equation:
CaCl2 (aq) + b Na3PO4 (aq)> c Ca3 (PO4)2(s) + d NaCl (aq)
iF I start with 3.50 g calcium chloride (and sufficient sodium phosphate), what is my theoretical yield of calcium phosphate? 
You would do well to post NEW questions as a new question and not tag it to one already answered. Why? Because we often check for new questions before we go back to the older ones; therefore, it COULD get overlooked.
Here is a worked example of how to calculate theoretical yield.
http://www.jiskha.com/science/chemistry/stoichiometry.html 
I am try to follow the example but lost.. cant figure out how they got that 420

I don't see a 420 in anything.
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