How many milligrams of sodium chloride should be used to compound the following prescription? (Note: Homatropine hydrobromide E-value: 0.17; 0.9% w/v NaCl is an isotonic solution).

RX

Homatropine hydrobromide  0.7 g

Purified water                         35 mL

Sodium chloride                       qs

Make isotonic sol. 

Sig. for the eye

 

Choose the best answer 

135 mg of NaCl

174 mg of NaCl

196 mg of NaCl

225 mg of NaCl

To determine how much sodium chloride (NaCl) is required to make this solution isotonic, we'll use the E-value (sodium chloride equivalent value) of Homatropine hydrobromide and the formula for an isotonic solution.

### Steps:

1. **Find the total amount of NaCl required to make 35 mL of an isotonic solution**:
   A 0.9% w/v NaCl solution is isotonic. This means that 0.9 g of NaCl is required for every 100 mL of solution.

   For 35 mL:
   \[
   \text{Amount of NaCl} = \frac{0.9}{100} \times 35 = 0.315 \, \text{g} = 315 \, \text{mg}
   \]

2. **Determine the contribution of Homatropine hydrobromide to the tonicity**:
   The E-value for Homatropine hydrobromide is 0.17. This means that 1 g of Homatropine hydrobromide is equivalent to 0.17 g of NaCl in terms of tonicity.

   For 0.7 g of Homatropine hydrobromide:
   \[
   \text{NaCl equivalent} = 0.7 \times 0.17 = 0.119 \, \text{g} = 119 \, \text{mg}
   \]

3. **Calculate the NaCl needed to make the solution isotonic**:
   The total NaCl needed for 35 mL is 315 mg, but the Homatropine hydrobromide contributes 119 mg of NaCl equivalency. So, the additional NaCl required is:
   \[
   315 \, \text{mg} - 119 \, \text{mg} = 196 \, \text{mg}
   \]

### Therefore, the best answer is **196 mg of NaCl**.

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Would you like further details or clarification on any step?

Here are 5 related questions:
1. How does the E-value influence the amount of sodium chloride needed?
2. What is the importance of isotonic solutions for ophthalmic prescriptions?
3. How would the calculation change if the E-value of Homatropine hydrobromide was different?
4. Why is 0.9% NaCl solution considered isotonic?
5. What are other common agents used to adjust tonicity?

**Tip:** When working with tonicity calculations, always verify the E-value of the active ingredient to ensure the accuracy of isotonic adjustments.

How many milligrams of sodium chloride should be used to compound the following prescription? (Note: Homatropine hydrobromide E-value: 0.17; 0.9% w/v NaCl is an isotonic solution).

RX

Homatropine hydrobromide 0.7 g
Purified water 35 mL
Sodium chloride qs
Make isotonic sol.
Sig. for the eye

Choose the best answer
135 mg of NaCl
174 mg of NaCl
196 mg of NaCl
225 mg of NaCl

Learn how to calculate the amount of sodium chloride required to compound an isotonic solution for ophthalmic use with Homatropine hydrobromide. This step-by-step guide covers E-values, isotonicity, and pharmaceutical calculations.

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