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Calculate Molarity(M) when 5g of NaOH is added in 200mL M/4 NaOH solution. Given...

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alphadav | Student, Undergraduate | Valedictorian

Posted June 24, 2011 at 3:22 PM via web

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Calculate Molarity(M) when 5g of NaOH is added in 200mL M/4 NaOH solution. Given Density of solution is 1.08 g/cm^3

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justaguide | College Teacher | (Level 2) Distinguished Educator

Posted June 24, 2011 at 3:44 PM (Answer #1)

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The 200 mL solution of NaOH with a molarity of M/4 has a density of 1.08 g/cm^3. The molarity of the solution is required after 5 g of NaOH is added to it.

To determine the resultant molarity we do not need the density of the solution. We need the molar mass of NaOH. Sodium hydroxide has a molar mass of 40 g/mole. 5 g of NaOH is equivalent to 5/40 = 1/8 mole.

The original solution contained 1/4 mole of NaOH per liter. 200 mL had 1/20 moles. On adding 1/8 mole, the total number of moles of NaOH in the 200 mL of liquid is 1/20 + 1/8 = 7/40.

This gives a molarity of (7/40)*1000/200 = 7/8

The resultant molarity of the solution on adding 5 g of NaOH is 7/8 M.

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