# Math Question needs solving...

#### poda521

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.

#### AnnaP

10+ Year Member
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.

It seems that you did everything right. You need to add 0.0054g of NaCl to make the solution isotonic. 0.0054g = 5.4mg of NaCl.You would not add NaCl in the form of NS, b/c that would change the volume of the solution! Just add 5.4mg of NaCl.

Are you sure the answer says 5.4mL? It should be 5.4mg, not mL.

#### poda521

Thanks so much. I was confused as hell with the answer being in mLs and I never thought logically that you'd add NaCl and not NS. (sighs) Seeing the answer in mLs and the word "isotonic" automatically swayed me to think of adding liquid.

Answer is in mLs but I guess it's an error on their part. I'm just glad my thought process is right on, just have to mind my units.

Thanks again.