December 15th - Percent Composition

And just before we relax, we had one more lesson to cover. It was a fairly quick one, and was all about percent composition. Though it is common to think that we were talking about the percentage of atoms in a compound, we were not! In fact, we were exploring the percent composition by mass.

Well, the best way to learn this is by doing an example. Let’s start off with the most common solvent, water. Water, with a chemical formula H₂O, has a molar mass at 18.0 g/mol. The hydrogen accounts for 2.0 g/mol, while the oxygen makes up the remaining 16.0 g/mol. Now, we can simply make a ratio of the element’s molar mass to the total.

Hydrogen: 2/18 = 11%

Oxygen: 16/18 = 89%

And there you have it! It’s as simple as that.

Let’s try percent composition with a twist. Let’s try to find the empirical formula of a substance. 


Ex. 1) An unknown substance is made up of:

·         2.04% H

·         65.3% O

·         32.65% S

Let’s assume we have 100g to work with. Therefore, we have

·         2.04g of H

·         65.3g of O

·         32.65 g of S

Now we need to find out how many moles of each substance we have.

·         Hydrogen = 1.0 g/mol = 2.04mol

·         Oxygen = 16.0g/mol = 4.08mol

·         Sulphur = 32g/mol = 1.02 mol

We can now make our empirical formula. Roughly, the ratio is H₂SO₄. And there you have it; sulphuric acid. Here's a video further explaining:

Posted by Andrew.

December 13th - Density and Moles

We're finally back from exams! Today, we learned all about density and moles! 

What is density, you ask? Well, density is a measure of mass per volume. In simpler terms, this means that it is a measure of how much matter exists in a given amount of space.

Look at this picture:

As you can see, there are a bunch of red particles jammed into the cubic metre of space. Let's estimate that there are 100 red particles, and each is equal to .01 kg. Density is equal to the mass divided by the volume, and the standard SI units used are kg/metre cubed. Therefore, the total density in the cube is 1 kg/metre cubed.

Now, look at this picture:

Here, there are many, many red particles jammed into the same cubic metre. Let's estimate there are now 10 000. If each has a mass of .01 kg, the total density is 100 kg/metre cubed. There is much more mass in the same volume, so it has a higher density!

In this chemistry course, we will be measuring density in the units of grams per litre (g/L) and grams per millilitre (g/mL). It is important to note that 1 mL is equal to a cubic centimetre, and 1 L is equal to a thousand cubic centimetres, not a cubic metre. 

Let's try some examples.

Ex. 1) A substance has a density of 10 g/mL. Determine the mass of 15 mL of the substance. 

We use dimensional analysis (with g/mL as our conversion factor) to solve this problem:

15 mL   x   10 g/mL   =   150g of the substance

We can also find out how many moles are present. Lets pretend that the substance is liquid nitrogen. Nitrogen has a molar mass of 14 g/mol.

150g   x   mol/14g   =   11 mol of liquid nitrogen

That was pretty easy. Let's kick it up a notch.

 Ex. 2) An unknown substance has a a molar mass of 96.0 g/mol. If 0.75 mol occupies a volume of 79 mL, determine the substance's density.

0.75 mol   x   96.0g/mol   =   72g of the substance

72g/79mL    =   0.91 g/mL of the substance

Ex. 3) Graphite has a density of 2.3 g/mL. The volume of graphite in a standard pencil is 0.44 mL. How many atoms of graphite are present in a pencil?

Graphite is made of carbon, so we can use the molar mass of carbon (12.0 g/mol) as a conversion factor.

0.44 mL   x   2.3 g/mL   =   1.012g

1.012g   x   mol/12.0g   =   .0843 mol

.0843 mol   x   6.02 x 10E23 atoms/mol   =   5.1 x 10E23 atoms of carbon

Fun, eh? As a review, here's a video that goes over the basics:

Posted by Michael.