I need some serious help with my Chem homework. I need to enter my answers into an online homework assignment generator, and it keeps prompting me that my answers are wrong, even though I've used the same steps for other problems. I have a few problems that I have worked out and keep getting the "incorrect answer" prompt. Can anyone please review what I did wrong and help me out? I would really appreciate it..

I need to calculate the number of molecules and calculate the mass (in grams) in Propane, C3H8

How do I do this? I know the molar mass is 44.11 correct? How do I calculate the number of molecules and mass without any given information? I keep entering 44.11 g as the mass but it says its incorrect...

I need to complete and balance the single replacement reaction:

I am given: Mg+HNO3

I put: Mg+2HNO3=2H+Mg(NO3)2 as a balanced equation but it says incorrect

Another single replacement:

Ba+HNO3

I put: Ba+HNO3=BaNO3+H