1. Potassium hydrogen phosphate is an ionic compound.
QUESTION:
Which of the following is correct formula for potassium hydrogen phosphate? A. KHPO4, B. K2HPO4, C. KH2PO4
Potassium hydrogen phosphate is an ionic compound.
Cation is potassium ion. K+
Anion is hydrogen phosphate: HPO42-
A compound is electrically neutral. The formula for an ionic compound should reflect the simplest set of ions with a total charge of zero. Choice A. K+, HPO42-; total positive charge = -1
Choice B. K+, K+, HPO42-, total charge=0
KH2PO4 is potassium dihydrogen phosphate
SCRIPT:
Which of the following is correct formula for potassium hydrogen phosphate? A. KHPO4, B. K2HPO4, C. KH2PO4
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
Potasisum hydrogen phosphate is an ionic compound….
You should know that if the name of a compound starts with the name of a metallic element it’s ionic.
Cations are generally derived from atoms of metallic elements, such as potassium.
Based on the location of potassium in the periodic table --- group 1A ---- we know that the charge of potassium ion is +1.
CLICK The first part of the name of any ionic compound is the name of the cation.
The rest of the name of the name of the anion.
In this case, it’s hydrogen phosphate.
And the formula of hydrogen phosphate is HPO4 with a –2 charge.
HIGHLIGHT HPO42-
How do we know this? It’s trivial. We look it up. Or better yet, we memorize it.
At this point, we can eliminate choices C.
In Choice C, the anion appears to be dihydrogen phosphate, H2PO4 with a –1 charge.
HIGHLIGHT H2PO4 in choice C
In fact, choice C would be the formula for potassium dihydrogen phosphate.
So, how do we decide between choices A and B? CLICK A compound is electrically neutral. The formula for an ionic compound should reflect the simplest set of ions with a total charge of zero.
Choice A would imply a total charge of -1. The formula in choice B implies a formula unit consisting of…
one cation, which has a charge of +1.
HIGHLIGHT K+
And one anion,
which has a charge of –2.
… +1 plus –2… equals –1
So, choice A is wrong.
How about choice B?
CLICK The formula implies that there are …
Two cations
HIGHLIGHT K+ , K+
And one anion
in one formula unit.
HIGHLIGHT charges as numbers are called
+1… plus +1… plus –2, equals zero.
Therefore, choice B is the correct formula for potassium hydrogen phosphate.
CLICK 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