Chemistry (Regents level)
posted by Jessica .
1 In the reaction Fe2O3 + 3 CO = 2 Fe + 3 CO2, what is the total number of moles of CO used to produce 112 grams of iron?
a) 1.0 b)2.0 c)3.0 d)4.0
2 Given the reaction:
4Al + 3O2 = 2Al2O3
How many moles of Al2O3 will be formed when 27 grams of Al reacts completely with O2?
a) 1.0 b)2.0 c)0.50 d)4.0
3 What is the molarity of a solution that contains 40 grams of NaOH in 0.50 liter of solution?
a) 1.0M b)2.0M c)0.25M d)0.50M
4 A 1kilogram sample of water will have the highest freezing point when it contains
a) 1 x 10^23 dissolved particles
b) 1 x 10^21 dissolved particles
c) 1 x 10^19 dissolved particles
d) 1 x 10^17 dissolved particles
Please don't just tell me the answers, please explain as much as you can.
Thanks in advance :)

1 In the reaction Fe2O3 + 3 CO = 2 Fe + 3 CO2, what is the total number of moles of CO used to produce 112 grams of iron?
a) 1.0 b)2.0 c)3.0 d)4.0
Convert 112 g Fe to mols. mols = grams/atomic mass. 112/55.85 = 2.00
Look at the equation. 2 mol Fe are produced for every 3 mols CO; therefore, the answer is 3.0.
2 Given the reaction:
4Al + 3O2 = 2Al2O3
How many moles of Al2O3 will be formed when 27 grams of Al reacts completely with O2?
a) 1.0 b)2.0 c)0.50 d)4.0
Convert 27 g Al to mols.
Convert mols Al to mols Al2O3 using the coefficients in the balanced equation.
27 g Al x (1 mol Al/27 g Al) = 1 mol Al.
1 mol Al x (2 mols Al2O3/4 mol Al) = ??
3 What is the molarity of a solution that contains 40 grams of NaOH in 0.50 liter of solution?
molarity = mols/L
mols = g/molar mass
molar mass NaOH = 40
mol NaOH = 40g NaOH/40 = 1 mol
molarity = mols/L = 1 mol/0.50 L = ??
a) 1.0M b)2.0M c)0.25M d)0.50M
4 A 1kilogram sample of water will have the highest freezing point when it contains
a) 1 x 10^23 dissolved particles
b) 1 x 10^21 dissolved particles
c) 1 x 10^19 dissolved particles
d) 1 x 10^17 dissolved particles
delta T = Kb*m*i
delta T is change in freezing point (from zero Celsius).
Kb is constant.
m = molality of solution
i = number of particles
The largest number will provide the largest number of particles and that will provide the largest delta T (which will be the lowest freezing point). Therefore, the highest freezing point must be from the solution containing the fewest particles and that appears to be answer (d).
Check my work. Check my thinking
Please don't just tell me the answers, please explain as much as you can.
Thanks in advance :)
In the future, tell us how you would work the problem. Most of us will gladly CHECK your answers (that's fairly fast for us) but few will provide detailed work to let YOU check yours (it takes us MUCH longer to do that.) 
mb