1. How many cations are in 2.00 mol Na3PO4?
QUESTION:
How many cations are in 2.00 mol Na3PO4?
A mol, B mol, C mol, D mol
One formula unit of sodium phosphate consists of cations (Na+) and 1 anion (PO43-)
Moles cations
3 mol Na+ =
Moles of f.u.
1 mol Na3PO4
How many cations are in 2.00 mol Na3PO4?
A mol, B mol, C mol, D mol
PAUSE
Subscripts in a formula are also counting numbers.
CLICK
In the case of sodium phosphate, one formula unit of this compound consists of 3 cations... There are three sodium ions indicated in the formula...
HIGHLIGHT 3 in Na3PO4
and one anion... There is one phosphate ion indicated in the formula
HIGHLIGHT PO4 in Na3PO4
CLICK Since a mole is just a group count, we can also say that one mole of sodium phosphate is made up of three moles of cations and one mole of anions.
Note that the official abbreviation for mole is em oh ell.... no E.
HIGHLIGHT mol
It should be obvious that the number of formula units and number of ions are directly proportional quantities....
When two quantities are directly proportional, their ratio is constant.
In other words, we can say that the ratio of
the number of cations
to the number of formula units
CLICK CLICK
is equal to
3 moles of sodium ions
to 1 mole of sodium phosphate units
We can rearrange this equation to solve for the moles of cations. Essentially, what we would end up doing is
multiply the given number of sodium phosphate units, 2.00moles
by the ratio of 3 moles sodium ions to one mole of sodium phosphate
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The ratio serves as a conversion factor.
SHOW CANCELLATION OF UNITS
The answer we get should be rounded off to three significant figures... since the numbers 3 and 1 in the conversion
factor are exact numbers. The least precise term in this calculation is 2.00, which has 3 significant figures.
TO summarize,
CLICK if we want to count ions in an ionic compound...
we simply multiply the number formula units by the subscript of the ion we’re interested in.
In the case we have here, the number of moles of sodium phosphate, is essentially just multiplied by the 3, which is the subscript of sodium.
...in order to get the number of moles of cations.
CLICK PAUSE END RECORDING
3 mol Na+
2.00 mol Na3PO4 x
= 6.00 mol Na+
1 mol Na3PO4
To count ions in ionic compounds: Ion count = Formula unit count x Subscript

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