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Percentages:

Solve these answers on your own paper, then scroll to the bottom to see how you do these!

  1. What is the percent silver in 15.0 g silver nitrate?
  2. What is the percent nitrogen in 25.0 g ammonium nitrate (NH4NO3)?
  3. What is the percent carbon in methane?
  4. What is the percent water in calcium nitrate, which is commonly found as a tetrahydrate?

Scroll down to see how to solve these!

 

 

 

 

More....

 

 

 

 


1.) What is the percent silver in 15.0 g silver nitrate?

First off, when doing problems dealing with percentages like this one, disregard the amount of substance. Remember, percent is per 100, meaning that the amount doesn't matter. Don't get it? Let's look at pizza.

Let's say you have 4 friends, and you decide to split a pizza. You order a pizza with 8 slices, and each of you gets 2 slices. This means you get 25% of the slices (2/8). However, you later decide to get a pizza with 16 slices (a large pizza, eh?). You then each get 4 slices. However, the percent of the pizza that you got stays the same (25%). Chemistry is the same way.

Ok, so back to the problem.

You know that silver nitrate is Ag(NO3). Finding the molar mass using your periodic table:

Ag 107.86
N 14.00
O3 15.99 * 3 = 47.97
Ag(NO3) 169.84 g

You can see that the molar mass is 163.84 grams. You also note that silver is 107.86 grams. So, to find percent silver in silver nitrate:

grams silver
-----------------------
grams total

=

107.86 g
--------------------
169.84

= 63.506 % (watch your sigfigs)

2.) What is the percent nitrogen in 25.0 g ammonium nitrate (NH4NO3)?

Again, you can discount the 25.0 g as irrelevant information (see above questions for explanation).

First, look up the molar mass of ammonium nitrate on your periodic table: (Note that ammonium nitrate has two nitrogens, four hydrogens, and three oxygens)

N 14.00 * 2 28.00 g
H 1.008 * 4 4.032 g
O 15.99 * 3 47.97
NH4NO3   80.00 g

Since we are finding the percent nitrogen:

grams nitrogen
-----------------------
grams total

=

28.00
--------------------
80.00

= 35.00 % (watch your sigfigs)

3.) What is the percent carbon in methane? (CH4)

This should be a breeze for you.

C 12.01 12.01
H 1.007 * 4 4.028
CH4   16.04

grams carbon
-----------------------
grams total

=

12.01
--------------------
16.04

= 74.88 % (watch your sigfigs)

 

4.) What is the percent water in calcium nitrate, which is commonly found as a tetrahydrate?

On first glimpse, one would think that calcium nitrate would be Ca(NO3)2. However, the problem also tells us that it is found as a tetrahydrate. A hydrate is a compound with H2O attached to it. So, this would be Ca(NO3)2-4H2O. A dihydrate would be Ca(NO3)2-2H2O. Get it? Good.

Now you just do what you've done in all the problems before!

Ca 40.08 40.08
N 14.00 * 2 28.00
O 15.99 * 10 159.9
H 1.008 * 8 8.064
Ca(NO3)2-4H2O   236.0

grams water
-----------------------
grams total

=

18.006 * 4
--------------------
236.0

= 30.52 %

 

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