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 sodium phosphate dibasic heptahydrate will you need to dissolve in 10 mL in order to obtain the osmolarity of 134.33? The molecular weight of sodium phosphate dibasic heptahydrate is 268 g/mol.
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lb equals 120 mg/10mL kg
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For sodium phosphate dibasic heptahydrate , the 'golden formula' is:
1 mmol= `[268]` mg = `[2]` mEq = `[3]` 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 134.33 mOsmol is contained in 1000 mL of solution, we keep the 134.33 mOsmol in the numerator of the second fraction for getting the 'mOsmol' part cancelled out.
Using dimensional analysis:
`(268 \quad mg)/(3 \quad mOsmol) × (134.33 \quad mOsmol)/(1000 \quad mL) × (10 \quad mL)/(10 \quad mL)= 120 \quad (mg)/(10 \quad mL)` Ans.