Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: webguy54 on March 07, 2011, 09:06:26 PM
-
Worked out a question, but can't figure out where I went wrong. Any help would be greatly appreciated.
Q) In order to administer a dose of 125mg of a poorly soluble drug with a relative mass (molecular weight) of 509 Da (g/mole) a solution is available of its salt (MW salt = 550 Da) in a 900 mg/L concentration. How many drops of the solution per minute must be infused over 20 minutes?
A) 150 drops/min
I calculated 128.5 drops/min. My work is below.
(509 g/mole) / (550 g/mole) = 0.925 g
0.925 g X (1mg/10^-3 g) = 925.45 mg
0.125 g X 0.925 g = 0.115625 g
0.115625 g X (1mg/10^-3 g) = 115.625 mg
Concentration (C) = 900 mg/L
Dose (D) = 115.625 mg
Volume = D/C = 115.625 mg / (900 mg/L) = 0.12847 L = 128.47 mL
128.47 mL X (20 drops / 1 mL) = 2569.4 drops
2569.4 drops / 20 minutes = 128.5 drops/min
-
Worked out a question, but can't figure out where I went wrong. Any help would be greatly appreciated.
Q) In order to administer a dose of 125mg of a poorly soluble drug with a relative mass (molecular weight) of 509 Da (g/mole) a solution is available of its salt (MW salt = 550 Da) in a 900 mg/L concentration. How many drops of the solution per minute must be infused over 20 minutes?
A) 150 drops/min
I calculated 128.5 drops/min. My work is below.
(509 g/mole) / (550 g/mole) = 0.925 g
Check your units, this is not a measure of mass, rather a measure of the molar mass ratio of drug to salt
0.925 g X (1mg/10^-3 g) = 925.45 mg
0.125 g X 0.925 g = 0.115625 g
0.115625 g X (1mg/10^-3 g) = 115.625 mg
I think you have this the wrong way round, you need more salt than drug on a mass basis, perhaps 0.125 g / 0.925
Concentration (C) = 900 mg/L
Dose (D) = 115.625 mg
Volume = D/C = 115.625 mg / (900 mg/L) = 0.12847 L = 128.47 mL
128.47 mL X (20 drops / 1 mL) = 2569.4 drops
2569.4 drops / 20 minutes = 128.5 drops/min
Where have you got the figure of 20 drops / ml from (not that it's unreasonable)?
S