Determine molarity from weight of drug and volume of the solution
4. Moles and Molarity 4.2) Molarity 4.2.1) Molarity - general
4.2.1.1) Molarity - general Normal 1
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55.63875 g of magnesium sulfate heptahydrate is dissolved in 250 mL of solution. If the molecular weight of magnesium sulfate heptahydrate is 120.3, then determine the molarity (M) of the solution
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lb equals 1.85 M kg
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Molarity is the number of moles (`n`) dissolved per liter of solution (`V`). Again, mole is the weight in gram (`g`) divided by the molecular weight (`a`).
In this example, the volume of the solution is 250 mL, or 0.25 L.
[1] Using formula method:
`M=g/(aV)=(55.63875\quad g)/(120.3\quad (g/(mol)) × 0.25\quad L)`
`therefore M = 1.85 (mol)/L=1.85` without unit, Ans.
[2] Using conceptual method:
First, count the number of moles (`n`) of magnesium sulfate heptahydrate present in this solution:
`=(weight\quad i n \quad g)/(mol ecu lar\quad wt)=(55.63875\quad g)/(120.3 \quad (g/(mol)))=0.4625 \quad mol`
Then, calculate how many moles will be present in 1L of solution. Using equation, we can write:
`(0.4625\quad mol e)/(250 \quad mL)=x/(1000 \quad mL) therefore x = 1.85` Ans.