1. Sodium phosphate is an ionic compound. Cation is sodium ion. Na+
QUESTION:
Which of the following is correct formula for sodium phosphate? A. NaP, B. NaPO3, C. Na3PO4, D. Na3(PO4)2
Sodium phosphate is an ionic compound.
Cation is sodium ion. Na+
Anion is phosphate: PO43-
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 D: Na+, Na+, Na+, PO43-, PO43- ; total charge=-3
Choice C: Na+, Na+, Na+, PO43-: total charge=0
SCRIPT:
Which of the following is correct formula for sodium phosphate? A. NaP, B. NaPO3, C. Na3PO4, D. Na3(PO4)2
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
Sodium phosphate is an ionic compound…. We know it’s ionic because sodium is a metallic element.
Cations are generally derived from metallic elements. In this case, so we know that the cation in this compound is sodium ion.
The first part of the name of any ionic compound is the name of the cation.
We should know that sodium ion has a charge of +1 based on its location in the periodic table. It belongs to group 1A.
The rest of the name of the name of the anion.
In this case, it’s phosphate. The formula of phosphate is PO4 with a –3 charge.
How do we know this? It’s trivial. We look it up. Better yet, we memorize it.
Chances are, your teacher expects you to memorize it.
At this point then, we can eliminate choices A and B. Neither choice has a PO4 grouping in the given formula.
CROSS OUT P in choice A and PO3 in choice B.
How do we determine the correct answer from choices C and D? 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. Let’s examine choice D.
The subscript for sodium is 3 and the entire formula for phosphate has a subscript of 2.
Each sodium ion has a charge of +1 and each phosphate ion has a charge of –3.
HIGHLIGHT as numbers are called
+1… plus +1… plus +1… plus –3… plus –3… equals –3
Therefore, the total charge implied in the formula is –3. Choice D is wrong.
If we look at choice C, we can see that the total charge is 0.
The formula given in choice C implies that there are 3 sodium ions for every phosphate ion.
Three times +1, gives us a total positive charge of +3.
One times –3, gives us a total negative charge of –3.
The overall charge is zero.
The correct answer is C.
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