Calculating Salinity parts per thousand

[CupcakeQueen]

I am doing the experiment "Can Water Float on Water?". I have a question regarding the calculation of salinity in parts per thousand notation.

Assume I dissolve 5 tablespoons of table salt into 3 cups of water. That would be 709.8mL and 91.25g. If I divide 91.25 by 709.8, I get .1285573 g/mL. That's the density, right? I don't know how to calculate the salinity. I want it in parts per thousand, but how can I compare grams to milliliters? I always thought that the density of water was constantly changing due temperature and pressure. Doesn't one milliliter of water equal one milligram only at 4 degrees C? My science teacher said something about a proportion.....please help. She also said something about boiling the water into the water evaporates and then weighing the salt. Thank you for reading.

[davidkallman]

Hi CupcakeQueen,

Welcome to the board!

In a previous post, barretttomlinson provided http://www.csgnetwork.com/h2odenscalc.html as a water density calculator.

The full discussion is at:
https://www.sciencebuddies.org/science- ... 43&p=18367

Note: the text above starting with "viewtopic" is clickable and includes http://

I don't know why it was abbreviated like it was.

[CupcakeQueen]

Yes, but I don't understand. How is that parts per thousand. I'm under the impression that salinity is expressed in parts per thousand. Is kg/m cubed parts per thousand? There must be some sort of mathematical way to determine the ppt.

[ChrisG]

Hi CupcakeQueen,
The usual, simple way to calculate parts per thousand of a solute in a solution is to express the concentration as mg of salt per g of solution (1 g = 1000 mg). That number gives you the parts per thousand by mass. So, if you have 0.13g/mL what is that in terms of mg/mL? If you assume (as most do) that 1 mL water = 1 g, then you will be able to find your answer in parts per thousand.

As you mentioned, water density changes slightly as the temperature goes up or down, but as you'll see from the calculator from David and Barret, that that change is very slight. Unless you need to work with a very high degree of accuracy and precision, the difference is negligible.

Good luck!
Chris

[CupcakeQueen]

So I would do amount of salt/amount of water multiplied by x/1000? I get it now.


Thanks to all of you for your help.

[ChrisG]

Yes, that is a decent approximation, though I should add that the calculation is usually expressed as the mass of solute (salt) divided by the mass of solution (salt + water) x 1000. And of course your units of mass need to be the same for that calculation to be correct.

As a side note, the "parts per thousand" unit of concentration is out of favor among some scientists because the meaning can be unclear. A person might wonder, is this parts per thousand by weight or by volume? To avoid that kind of confusion, some scientists favor expressing concentration in terms of molarity (http://en.wikipedia.org/wiki/Concentration#Molarity) or by using explicit units such as mg/L.