[Histonet] (no subject)

[Lee & Peggy Wenk]

An anhydrous chemical has NO water attached to it. A trihydrate chemical has 3 waters attached to it.

This becomes important when making solutions, as the weight of the water has to be included in the weight of the compound. 

100 g of sodium acetate, trihydrate is NOT the same amount of sodium acetate as 100 g of sodium acetate, anhydrous. Some of the weight of the sodium acetate trihydrate is the water.

The molecular weight of sodium acetate anhydrous is about 82. 
(CH3COONa) = (2 Carbons x 12) + (3 Hydrogens x 1) + (2 Oxygens x 16) + (1 Sodium x 23)
(You need to look at a periodic table to get the weights of the chemicals, and the side of the container to get the formula. Often, the side of the container also lists the molecular weight.)

The molecular weight of sodium acetate trihydrate is about 136.
(CH3COONa*3H2O) = weight of the anhydrous (82) plus the weight of 6 more hydrogens (+6) plus 3 more oxygens (+48)

So, if the directions calls for 100 g of the anhydrous sodium acetate, but you only have trihydrate sodium acetate, you need to set up a ratio:

x = (136/82) x 100 = 165.8 g 

This means that 165.8 g of sodium acetate trihydrate to equal 100 g of sodium acetate anhydrous. So to make up the solution, you would use the 165.8 g of sodium acetate trihydrate, instead of the 100 g.

Hope this helps.

Peggy A. Wenk, HTL(ASCP)SLS
William Beaumont Hospital
Royal Oak, MI 48073

  ----- Original Message ----- 
  From: JCarpenter764 <@t> aol.com 
  To: histonet <@t> lists.utsouthwestern.edu 
  Sent: Monday, October 27, 2003 9:32 AM
  Subject: [Histonet] (no subject)


  what is the difference between the anhydrous compound and the trihydrate compound? 
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