Have a RX to compound a 3% substance, qs to 3ml. E = 0.24 for the substance. How much sodium is needed to to make compound isotonic? Answer says I need 5.4mls. Below is what I did, but I'm sure I'm tackling this problem wrong...which is where SDN comes in. 1) 3% x 3ml = 0.09g of substance in compound. 2) 0.09g x .24 = 0.0216g of Na equivalent. 3) C1V1=C2V2 to find that I need 0.027g of Na in the 3ml compound to make it isotonic. 4) 0.027g Na needed - 0.0216 already in compound = 0.0054g of Na needed to be added to make compound isotonic. 5) 0.0054g x A = 0.9g x 100ml = 0.6mls of NS needed.