Stoichiometry

C₃H₈ + 5 O₂  3 CO₂ +4 H₂O

How many grams of O₂ are needed to react completely with 0.731 g of C₃H₈?

i dont know the formula to use

A student reacted 25 mL of 3 molar lead (II) nitrate with 4 grams of sodium sulfide.

a) Write a balanced equation 

- Finished balanced equation

b) How many moles of lead (II) nitrate are in the reaction?

- I think you use stoic... But I really don't know... :(

c) How much sodium nitrate will be produced (grams)?

d) Identify both the limiting reagent and the excess.

e) How much excess is present (grams)?

I'm having trouble because my chemistry teacher doesn't really show up half the time. So my question is if I can get some help setting up the problem on this assignment. 

The question is:

You used 25mL of 6.0M NaOH to neutralize 100mL of an unknownown acid in a titration experiment.What is the molarity of the acid?

how many grams of sodium chloride are formed from 17.4g of sodium nitrate?

When a mixture of sulfur and metallic silver is heated, silver sulfide is produced. What mass of silver sulfide is produced from a mixture of 3.0 g Ag and 3.0 g S8?

I have balanced the equation: 16Ag(s) + S8(s) > 8Ag2S(s)

I converted the Ag into moles but not the S because do I times the mass of S by 8?

I am lost as to how to solve, what formula do I use because I do not think it is a limiting reaction problem 

Thanks

solution A was prepared by extracting K2CO3 from 8g of potash and making  1.00dm^3 of solution with pure water. Use your answers from question 5 ( concentration in gdm^-3 of K2CO3 in solution A = 6.16 gdm^-3) to calculate the percentage by mass of K2CO3 in the sample of potash used to prepare solution A.

Using this equation:

H₂O₂ (aq) + 2 H⁺ (aq) + 2 e^- ---> 2 H₂O

How many moles of HCl will just react with 0.424 g Ba(OH)2?

2HCl + Ba(OH)2 ----> BaCl2 + 2H2O