# I need help in my chemistry homework

skyblueff0
okay im gonna give you like 4 problems out of like 22 i need to do for homework before the breaks is over can someone just tell me what formula to use, or it would be even better if someone can show me

" "- subscript numbers

1. Determine the mass of lithium hydroxide produced when 0.38g of lithium nitride reacts with water according to the following equation: Li"3"N + 3H"2"O --> NH"3" + 3LiOH

2.What mass of hydrogen peroxide (H"2"O"2") must decompose to produce 0.77g of water?

3.Find the mass of benzene (C(6)H(6)) required to produce 2.66 L of carbon dioxide gas at STP from the reaction described by the following equation : 2C"6"H"6" + 15O"2" --> 6H"2"O + 12CO"2"

4.Determine the mass of sodium nitrate produced when 0.73g of
nickel(II) nitrate reacts with sodium hydroxide according to the following equation: Ni(NO"3")"2" + 2NaOh --> Ni(OH)"2" + 2NaNO"3"

## Answers and Replies

Monique
Staff Emeritus
Gold Member
Originally posted by skyblueff0
okay im gonna give you like 4 problems out of like 22 i need to do for homework before the breaks is over can someone just tell me what formula to use, or it would be even better if someone can show me
We have a policy that we don't do your homework for you :) here's some hints:

1. Determine the mass of lithium hydroxide produced when 0.38g of lithium nitride reacts with water according to the following equation: Li"3"N + 3H"2"O --> NH"3" + 3LiOH
Not too difficult, first look up or calculate the molecular weight of Li3N (mw), then compute the number of moles Li3N that you have, you get three times the number of moles LiOH out of the reaction, convert back to grams. mw is in grams/mole, so if you know the number of grams, you divide by mw to get the number of moles (and vice versa).

2.What mass of hydrogen peroxide (H"2"O"2") must decompose to produce 0.77g of water?
First determine how H2O2 reacts with water, and do the same as above.

3.Find the mass of benzene (C(6)H(6)) required to produce 2.66 L of carbon dioxide gas at STP from the reaction described by the following equation : 2C"6"H"6" + 15O"2" --> 6H"2"O + 12CO"2"
Same thinking here, keep track of the reaction ratios (2 C6H6 produce 12 CO2), at STP a mole of any ideal gas occupies 22.4 liters.

4.Determine the mass of sodium nitrate produced when 0.73g of
nickel(II) nitrate reacts with sodium hydroxide according to the following equation: Ni(NO"3")"2" + 2NaOh --> Ni(OH)"2" + 2NaNO"3" [/B]
Again, same thing.

Last edited:
If your chemistry textbook is any good at all, there should be stoichiometry problems similar to the ones you are working.
H2O2 yields H2O + O2 and you'll have to balance the equation first.
Here's a little trick for conversions:

GMD = grams to moles, divide
MGM = moles to grams, multiply

LMD = liters to moles, divide (22.4L)
MLM = moles to liters, multiply (22.4L)

Monique
Staff Emeritus
Gold Member
Very good tip:

I always have the following picture in my mind:

Code:
          * mw              * 6.022 x 10^23
grams     <--       moles      -->         molecules

|
|* 22.4L (at STP)
V

liters
It is all very simple, just remember that picture and the fact that you ALWAYS have to multiply when you go from the mole to any of the other measurements. And when you want to convert something into a mole, you ALWAYS divide.

Hope that makes things clearer