1. K2CO3 is an ionic compound. Cation is potassium ion: K+
QUESTION:
Which of the following is correct name for K2CO3? A. dipotassium carbon trioxide, B. potassium carbide C. dipotassium carbonate, D. potassium carbonate
K2CO3 is an ionic compound.
Cation is potassium ion: K+
A compound is electrically neutral. Overall charge=0. Total positive charge: (+1) x 2 = +2 Total negative charge: -2
Therefore, anion is carbonate: CO32-
SCRIPT:
Which of the following is correct name for K2CO3? A. dipotassium carbon trioxide, B. potassium carbide C. dipotassium carbonate, D. potassium carbonate
PAUSE
To determine the formula of a compound given its name, we need to first classify the compound, then apply the rule for that type of compound.
CLICK
K2CO3 is an ionic compound…. We know this because potassium is a metallic element.
When atoms of metallic elements form compounds, they typically end up as positively charged ions
CLICK In fact, we know that potassium, which belongs to group 1A, is always observed to form ions with a charge of +1.
HIGHLIGHT K+
To name an ionic compound, we simply state the name of the cation, then the name of the anion.
At this point, we can eliminate choices A and C. The name of the cation is potassium, not dipotassium.
CROSS OUT dipotassium in choices A and C
It doesn’t matter that there is a subscript of 2 for potassium in the formula. When naming ionic compounds, we do not use prefixes.
To continue…let’s figure out the anion.
CLICK We use the fact that a compound is electrically neutral. Therefore, the total for all ions indicated in the formula must be zero.
CLICK We know that each potassium ion has a +1 charge HIGHLIGHT (+1)
Since there are two potassium ions given in the formula K2CO3 HIGHLIGHT subscript 2
the total positive charge is +1 times 2, or +2
CLICK Therefore, the total negative charge must be –2.
CLICK Therefore, the anion must be CO3 with a –2 charge. This anion is called carbonate.
How do we know it’s called carbonate? We look it up, or memorize it. Compounds containing carbonates are quite common, so it’s best
to just memorize that carbonate ion has a formula of CO3 with a –2 charge.
So, now, we can eliminate choice B. The anion is carbonate, not carbide.
CROSS OUT carbide
The correct answer is D. The name of the compound whose formula is K2CO3… is potassium carbonate.
CLICK The name of the cation is potassium… and the name of the anion is carbonate.
PAUSE END RECORDING

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08