[webguy54]

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

[sjb]

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