Dont Understand Reconstituting Medications Questions

Question

Erythromycin Ethylsuccinate oral suspension bottle instructs you to add 53 mL of water and shake vigorously. This makes 100 mL suspension for a concentration of 200mg per 5mL. Refrigerate after mixing.

pt care orders read: give 550mg of Erythromycin Ethylsussiante by mouth every 8 hours. How much will you administer?

Ceftriaxone 1 g reconstituting options, option 1: add 3.6 mL of sterile water to obtain 250mg/mL concentration

Ordered Ceftriaxone 750 mg IV q 6 h How much will you draw up?

Ordered Ceftriaxone 450 mg IV q 4 h How much will you draw up?

Answer 1

Simple ratios can help here:

g substance _original = g substance _new

mL liquid _original mL liquid _new

The proportion of g/mL remain the same, so you have

200mg = 550mg

5mL ?? mL

Cross multiply to solve: 200 * ?? = 5 * 550

Similarly, the other two:

250mg = 750mg

1 mL ?? mL

250mg = 450mg

1 mL ?? mL

There seems to be a lot of extraneous information in this question -- the frequency of administration is not relevant to the total volume of a single dose. I don't think the reconstitution directions are necessary either, since the directions clearly give the final concentration as long as you use the correct volumes of water.