# 5 questions: 1. How many moles of neon occupy a volume of 14.3 L at STP?

2. A sample of nitrogen gas had a volume of 500. mL, a pressure in its closed container of 740 torr, and a temperature of 25°C. What was the new volume of the gas when the temperature was changed to 50°C and the new pressure was 760 torr?
3. What is the molarity of a solution which contains 58.5 g of sodium chloride dissolved in 0.500 L of solution?
4. What is the molarity of a KCl solution made by diluting 75.0 mL of a 0.200 M solution to a final volume of 100. mL?
5. For Kw, the product of [H3O+] and [OH-] is what?

Answer:
For questions 1&2 you need to use the ideal gas law which is PV=nRT where P is pressure, V is volume, n is the number of moles of gas, R is a constant and T is temperature. STP is a pressure of 1 bar and a temperatue of 273 K. At STP there is 1 mol of gas is 22.4 L, thus to get the number of moles just take 14.3/22.4 and you will get the number of moles to be .638 mol.
2. When you have a problem like this, you set the things that change equal to eachother because they should be equal to a constant. You then solve for the unknown. So in your problem pressure, temp and volume change and your number of moles stays the same. So you would write (Pi*Vi)/(Ti)=(Pf*Vf)/(Tf). You then solve for Vf so your final calculation will be (740 torr * .5L * 323K)/(760 torr * 298K), your answer will come out in L. It is important that you do the calculation using temperature in kelvin and not in celsius. You should get a volume of 0.53 L
3.Molarity is equal to mol/Liters. Number of moles is equal to mass of sample/molar mass. The molar mass of KCl is 74.55, so you have .785 mol of KCl. You then take that amount and divide it by 0.5 L and get 1.57 M as your molarity.
4. Again, M=mol/L, so first calculate the number of moles you have in the 75.0 mL solution by taking 0.200*0.075L=0.015 mol. You then take that number and divide it by the new volume so M= 0.015 mol/0.100 L, thus your molarity is 0.150 M.
5. I'm not really sure what you are asking. Kw is always equal to 1*10^-14. What you have written is the concentration of acid and the concentration of base in a solution. A neutral solution has a pH of 7 which means that [H3O+] =1*10^-7. There is a process called the autoionization of water which is that 2H20 <---> H3O+ + OH-. Thus, the concentration of H3O+ is equal to the concentration of OH-. So, Kw = [H3O+]*[OH-]= (1*10^-7)*(1*10^-7). I hope that helps, again, I'm not sure I understand the question.
All answers given have the correct number of sig. figs if that is something your teacher pays attention too.
Well for 1, just divide 14.3 L by the ideal gas constant of 22.4 L/mol.

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