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Chemistry question

I'm attempting to understand a little more about calculating dosage and 
concentrations of various chemicals.

I've already figured out how to determine (I think) what percentage of a
compound is a certain element.   For instance, while trying to come up with
the right values for Potassium Sulfate, i figured:

Potassium is 39 grams per mole.   Since it's 'K2', I figured 2x39 = 78.

Adding up the remaining parts (SO4), S=32g/mole, O=16g/mole, so
32 + (4x16) = 96.   If I understand what these numbers mean, one mole of
K2SO4 weighs 174 grams.   And since the 'K2' is 78 g/mole, then 45% of K2SO4
is potassium.

I did the same procedure for KNO3, and the values I calculated matched those
listed on TheKrib (http://www.thekrib.com/Plants/Fertilizer/dosing.html)

So, I think I can figure out what percentage of any given compound is comprised
of a given element.   Pretty simple.    And, I think I'm right in saying that
1 gram of an element dissolved in 1 liter of water will create a concentration
of 1000ppm.    So, if I dissolved 1 gram of K2SO4 in 1 liter of water, I'd have
a potassium concentration of 529ppm (not counting any potassium already present 
in the water..).     From there, it's simple to calculate how much a given
dosage of that solution would add to a tank.

BUT!  Now I'd like to be able to measure out the K2SO4 in teaspoons or some
similar volume measurement.

Looking at that page on TheKrib dealing with dosing, there is a chart that shows
an approximate weight per 1/4 teaspoon of various compounds.    But, it doesn't
list K2SO4.   I know I could just weigh out a known volume of K2SO4, but is there
a way to calculate it based on basic chemistry information?   For some elements,
I can find "Density of solid".  But for Chlorine, Oxygen, etc (gases), it doesn't
list the value.    I also have a Molar Volume (cm3) which I assume means one mole of
an element would take up xx cubic centimeters.   I thought I could use that info to
figure it out, but my numbers would not match up to any of the grams per 1/4tsp
on the page at TheKrib.

So, first, an I missing some important concept on the first part dealing with
percent of compound, and concentrations?    Next, what am I missing regarding
calculating mass per given volume (or volume per given mass, I'm not picky...)

This is mostly a simple academic excersise, but I like to have an understanding
of the various charts and references that I use regularly.

Chuck Gadd