How do I solve these? Final tomorrow, help please!

Can you explain to me in simple terms how to do one or more of these:

  • Find molecular formula from empirical formula

  • How to find how many grams of (element) can be produced from (grams of element) with a balanced equation
    With this one I am asking how to set the ratios. This is what I do, is my method correct?
    EXAMPLE:
    (X means an element given like Cl or N or O, etc
    Y means second element given)

element X grams 1 mole X Ratio Y 1
----------------- * ---------- * ------------ * ---------- = How much Y? IDK
1 MM Ratio X MM

So I would go about doing this and I dont know if im doing it correctly

An actual PROBLEM I need help answering (on study guide):

-Determine the empirical formula of a compound that contains 69.5% oxygen and 30.5% nitrogen

1 Answer
glerp · Truong-Son N.
Jan 30, 2018

For determining the empirical formula of a compound that contains 69.5% oxygen and 30.5% nitrogen, and then its molecular formula.
The bolded parts are there so it will be easier to remember steps.

Explanation:

1. If you assume you have 100g of the compound , you will have 69.5g of oxygen and 30.5g of nitrogen.

2. You can now convert to moles using the molar mass of each element, given in the periodic table, using the formula

#n=m/M# which means moles = mass / molar mass .

For oxygen:

#n=(69.5g)/(16g/(mol))#

#n=4.344 mol#

For nitrogen:

#n=(30.5g)/(14.01g/(mol))#

#n=2.179 mol#

#3#. Divide the number of moles for all elements (values from the second step) by the smallest value (# of moles) which in this case would be 2.179 mol.

For oxygen:

#(4.344 mol)/(2.179 mol) = 1.994#

For nitrogen:

#(2.179 mol)/(2.179mol) = 1 #

#4.# Round both values in step 3 to the nearest whole number . You will get #2# for oxygen and #1# for nitrogen. These are the subscripts for the empirical formula.

Therefore the empirical formula for this is #NO_2#.

To find the molecular formula, you should be given molar mass for the whole compound. Then you can calculate molar mass of the empirical formula , and divide.

molar mass of molecular formula / molar mass of empirical formula

Suppose you have a molar mass that is twice that of the empirical molar mass. Multiply the subscripts of the empirical formula by 2. This will be the molecular formula of #N_2O_4#.

Examples: http://www.chem.tamu.edu/class/fyp/stone/tutorialnotefiles/empirical.htm

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