Chemistry. Can someone explain some conversions

[dpatte]

Im trying to find out is how I convert 1 teasppon of NaCl / litre into
ppm and % (based on mass).

To get %, do I need the atomic weight of the elements, and how does this relate to moles?

[darkdep]

Have you tried the OVAS calculators here:  http://ovas.ca/index.php?page=81  ?

[dpatte]

no, i havent, but i guess im asking a technical question first so i will know how to use the calculators.

Simplifying my question, is a clump of NaCl considered:

- 50% sodium, 50% chlorine (is the calculation based on ion percentages), or are the components adjusted according to their atomic weights, giving the relative mass of each elemnt?

- Also, is PPM is based on ion ratios, or relative mass?

[darkdep]

NaCl means the molecule has one atom of Sodium and one atom of Chlorine; as to how much this molecule weighs (molecular weight), you would need to look up the atomic weight of each of those.

PPM is based on volume; i.e in order to figure that out, you split your volume of water up into a million units of whatever size, and measure accordingly.  i.e if you have 20ppm of NaCl in water, that for your water amount (say, 100gallons) it means if you split 100gallons into 1 million equal amounts, 20 of them would be pure salt.

[jbbeer]

Here is my understanding of it. % and ppm are measures of concentration by weight - g/100ml and mg/L, respectively. A clump of NaCl is 50% Na and 50% Cl in a molar sense. There are an equal number of Na and Cl ions in the clump. By weight however, there is more Cl because it has a higher molecular weight (35.45) vs. Na (22.99).

example: a 10% NaCl solution is 10g/100ml, or 10000mg/0.1L=100000ppm, or (10g / 58.443g/mol) / 0.1L = 1.71 mol/L

I'll admit that ppm confuses me and the above definition could be wrong. People in my workplace tend to use it so I looked it up and there are several definitions of this unit - by mass (mg/L), by volume (as DarkDep pointed out), and atomic (moles of substance per million moles water). I'm going to stick with molar concentrations! 

[artw]

Ah Google
http://www.saltinstitute.org/teaspoon.html

[dpatte]

i still dont get it.

if i want to raise my water from 0 ppm to 20ppm of salt, how would I calculate how much volume of salt i should add?

how do i measure the right volume to add?

but if ppm is measured in mass - i m quite sure I could work out a way of meaururing 20/1000000 of the mass of the water and adding that same mass of salt.

[dpatte]

the more i think about it - the more i think its based on mass (weight)

if you want to add 20 PPM of salt to 100 US gallons of water

1) convert to litres = 378 litres of water
2) convert to mass = 378 kg of water
3) convert to grams = 378 000 g of water
4) calculate required salt = 20/1,000,000 * 378,000 g = 7.6 g

This works out to about 1/4 ounch of salt

I think this gives 20PPM of NaCl by weight (mass), but since Na and Cl have different masses, there would be about 12PPM of Na ions, and 8 PPM of Cl ions - by mass.

does this sound right?

Then I'd have to do some more math and compute the pH if i wanted to know the PPM by ion conentration.

[BigDaddy]

PPM is equal to milligrams per litre.

As far as chemical compounds go, you can usually google the compound to get the % of each element in said compound.

For example, KNO3 is roughly a 60/40 split of K and NO3.

I think you want something simpler, though.. you just want to increase your salt levels by 20ppm

So, that would be 20 milligrams per litre of water.  Or roughtly 80 milligrams per gallon.

[artw]

80 milligrams per gallon is funny, thats a combination of 2 units, it should be something like 0.00282191696 ounces per gallon.   Ahh, metric

[jbbeer]

Yes, 7.6g in 378L of water sounds right. You are adding Na and Cl ions in equal amounts. Do you really need to be concerned about the absolute mass of Na and Cl in the tank?

[dpatte]

I just looked it all up on wikipedia about PPM

there are two common meanings for PPM for chemicals in a solution.

PPM by mass (or weight) in which case you measure the mass of your water vs the mass of your chemical you add.

or PPM by mole , in which case you compute the relative number of ions of each chemical.

I would think that most measurements in books are the first - so as bigdaddy says, and as I calculated - its milligrams / litre .

[darkdep]

Oh dood, just throw in 1 teaspoon per 5 gallons and fahgeddabouddit

[freshwater]

Hello David, I'm really happy with the impeller I got from you, so I thought I'll help you out on this one.

Let's see how much I can remember from first year chemistry. 


1 teaspoon = 4.92892161 ml
Density of pure NaCl = 2.16165g/ml at 27 degree C
1 teaspoon NaCl = 10.65g

Molar mass of NaCl = 58.44 g/mol
Therefore, # of mole in 1 teaspoon of NaCl = 10.65/58.44 = 0.1822 mol

Density of pure water (H2O) = 1g/ml = 378.52kg/100 gallon
Molar mass of H2O = 18.02g/mol
Therefore, # of mole in 1 liter(1000 ml) of H2O = 1000/18.02 = 55.49 mol
# of mole in 100 gallon(378.52 liter) of H2O 55.49 x 378.52 = 21004 mol

ppm = parts per million (I pretty sure it just means how many molecules of one element or compound in one million molecules of the solution, but don't quote me on this)

So, let X be the # of mole of NaCl needed to produce a concentration of 20ppm,

X / 21004 = 20 / 1000000
X = 20 / 1000000 x 21004
X = 0.42008

0.42008 mol x 58.44 g/mol = 24.55g

24.55g / 2.16165g/ml = 11.36 ml

11.36 / 4.92892161 ml / teaspoon = 2.3 teaspoon

Looks like you need to add 2 ¼ - 2 ½ teaspoon of salt to the 100 gal tank to get 20 ppm


To save time, I found all the conversion factor online, so they r not totally reliable, but this should be a pretty good estimate.

[Quatro]

24.55g x 2.16165g/ml = 11.36 ml

11.36 x 4.92892161 ml / teaspoon = 2.3 teaspoon

Looks like you need to add 2 ¼ - 2 ½ teaspoon of salt to the 100 gal tank to get 20 ppm

Very impresive Freshwater.  I agree with your answer but in the last 2 lines you use an X (multiply) but you really meant / (divide).  Good work.

Mike S

[freshwater]

Good call Mike, it has been fixed.

Zi Z.