Type 2: Find the weight of the solute from mOsmol and volume

5. mEq and mOsmol 5.2) mOsmol 5.2.1) mOsmol - General

5.2.1.1) mOsmol - General Normal 1


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How much phenytoin sodium will you need to dissolve in 50 mL in order to obtain the osmolarity of 138.69? The molecular weight of phenytoin sodium is 274 g/mol.


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lb equals 950 mg/50mL kg

    For phenytoin sodium , the 'golden formula' is:
    1 mmol= `[274]` mg = `[1]` mEq = `[2]` mOsmol.

    Osmolarity is the number of mOsmol per 1 L (1000 mL).
    Since we need mg quantity in the numerator of the desired unit, we begin with keeping it in the numerator of the first fraction.
    Again, since 138.69 mOsmol is contained in 1000 mL of solution, we keep the 138.69 mOsmol in the numerator of the second fraction for getting the 'mOsmol' part cancelled out.
    Using dimensional analysis:
    `(274 \quad mg)/(2 \quad mOsmol) × (138.69 \quad mOsmol)/(1000 \quad mL) × (50 \quad mL)/(50 \quad mL)= 950 \quad (mg)/(50 \quad mL)` Ans.


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