The past couple of days have proven to be a great start to the third trimester of our school year. I’m very excited for all of you — and to give you a word of encouragement — there were no failures for second half from T2! Let’s try to keep that trend!!
We got our notebooks set up for the new trimester. Additionally I got each student a hanging folder to hold his/her old work from their first trimester with me. They will be able to use this work to help study for their final exam in May. They have access to any of their old work at any time, if they wish.
We also began our work for T3 with a little review of molar mass and naming compounds. Guys — you need to make sure you understand the nomenclature flowchart and can use your polyatomic ion list! Both documents are linked to this site, and they will be posted to the class website as soon as they get updated. Remember to use the correct prefixes when you have hydrated compounds.
An example of how to find the molar mass of a compound is as follows:
Find the molar mass of KMnO4.
K 1 x 39.10 = 39.10
Mn 1 x 54.94 = 54.94
O 4 x 16.00 = 64.00
Total is 39.10 + 54.94 + 64.00 = 158.04 g/mol. This is the molar mass of potassium permanganate.
Remember — for a hydrate, the process is the same, we just treat the molecules of water as an “element” of their own, with a molar mass of 18.02.
Today we took molar mass one step further and talked about percent composition. DOn’t forget, the numerator of the fraction must be the mass of the “element” AFTER multiplying by its subscript (or coefficient, if talking about the water attached to a hydrated compound).
Let’s look at the example above.
Find the percent composition of each element in potassium permanganate.
%K = (39.10/158.04) x 100 = 24.7% K
%Mn = (54.94/158.04) x 100 = 34.8% Mn
%O = (64.00/158.04) x 100 = 40.5% O
Remember — you can check your work by adding the percents to make sure they total 100 (or VERY close to 100).