PETER Z/mixing sulfite solutions

[scotty]

I have been trying to learn about molar solutions so i can make a .1 solution to use in my acid tests and as a substitute for campden tablets.


((Dont laugh but a few months back when my gf went to philadelphia i asked her to get me a large package of sodium metabisulfate crystals. I told her to take whatever Barry suggested. I now have 10 pounds of the stuff.))
So you shouldeasily understandwhat sparked my interest in finding ways to use it.




I will condense what i have read
One article stated that a .1 solution is 10 grams in 100ml of water.
((( definitely wrong)))




As i studied more, i came across a formula that uses molar mass/FW---




Im stopping here because i just realized that i did not see what seem to be division signs and must study the formula further.

JUST A NOTE---Im using a formula that i am not sure of but come up with 1.9 grams for 100ml of water.




This seems to be correct as i study moreEdited by: scotty

 

[scotty]

After more research, see thar the 10 grams per 100ml is not what i am looking for.
THE FOLLOWING IS THE PROPER FORMULA


Molarity is the unit used to describe the number of moles of a chemical or compound in one liter (L) of solution. By this definition, a 1.0 Molar (1.0 M) solution is equivalent to one formula weight (FW = g/mole) of chemical dissolved in 1 liter (1.0 L) of solvent (usually water). Formula (or molecular) weight is always given on the label of a chemical bottle.
<DIR>


Example 1: To prepare a liter of a simple molar solution from a dry reagent


Multiply the formula weight (or MW) by the desired molarity to determine how many grams of reagent to use:
Chemical FW = 194.3 g/mole; to make 0.15 M solution use

194.3 g/mole * 0.15 moles/L = 29.145 g/L</DIR>
The FW of sodium metabisulfate is ((190.1.-Still did not prove it myself))


So I think my result of 1.9 grams per 100 mlmay becorrect.


ANYONE Please correct me if i am mistaken



Edited by: scotty

 

[PeterZ]

Sodium metabisulfite is Na2S2O5. The formula weight is calculated by adding up all of the molecular weights of the components (I normally round off to the nearest half MW - unless you are working with a volumetric flask and everything is at 20C this is more than accurate enough).

Na = 23 x 2 = 46
S = 32 x 2 = 64
O = 16 x 5 = 80

46 + 64 + 80 = 190

Thus, 190 grams of sodium metabisulfite added to 1 L of water would be a 1 Molar solution. Now I doubt that the meta that you have is CP (Chemically Pure - 99.9% or better). It's probably about 95%, with most of the balance being sodium sulfate (Na2SO4). If we assume 95% purity (you can buy CP from lab supply houses, but be prepared to pay through the nose) then:

190 / .95 = 200 g/L for 1 1M soultion.

To make 100ml of a 0.1M solution the 1M - 1L quantity (200g) x .1 M x .1L = 2g.

Now, how are you going to measure 2g? Most electronic kitchen scales measure in 5g increments. Even if the scale is accurate to 1g then everything from 1.51g to 2.49g will read as 2g. Your best bet is a reloader's scale available from most gun shops. These read in grains Avoirdupois. There are 7000 grains in a pound, and they are precise to 0.1 grain. The last time I bought one (about 15 years ago) I paid about $20 for the RCBS brand.

There are 454g in a pound, and 7000 grains. Thus we have:

2g / 454 g/lb x 7000 gr/lb = 30.83 grains - call it 30.8 grains.

Now, to do all this at the accuracy you want you will really need the following:

Distilled water, boiled for twenty minutes and cooled to 60C.
CP sodium metabisulfite: ~$30/100g from a lab supply house.
Metller balance: used reconditioned ~$12,000, new ~$25k
100 ml volumetric flask ~$30

Vibration free table for the balance - priceless.

 

[scotty]

15.432 grain per gram--i will use my OHAUS gunpowder scale.
15.432 X2 = 30.8 grains. i can measure to 1/10 0f a grain.


Thanks for the tip on the purity i will adjust my 1.9 figure to 2 grams.Edited by: scotty