iPad 10:14 PM * 50% X Exp. 8 - Prelab - Equivalent Mass and Ka of an Unknown Aci

Question

iPad 10:14 PM * 50% X Exp. 8 - Prelab - Equivalent Mass and Ka of an Unknown Acid: Attempt 1 Status: 2.00000 points Save Answer UESTION 1 In a different titration (not the one above), 1.263 g of an unknown monoprotic acid required 16.11 mL of 0.199 M sodium hydroxide to reach the equivalence point. What is the molar mass of the acid? 2.00000 points Saved UESTION 2 Why does the sodium hydroxide solution need to be standardized? Sodium hydroxide absorbs and reacts with gases from the atmosphere changing its concentration over time You can never trust the prep TA who made the solution, so it is always a good idea to check the concentration. O The sodium hydroxide undergoes decomposition over time so an accurate concentration must be determined every use. Readings on a buret in a tiration are more accurate than those from a balance, so in general, it is always better to perform a titration than use mass to calculte concentrations.

Explanation / Answer

Q1. At equivalence point number of moles of acid is equal to the number moles of basse i.e.

Number of moles of acid (nA) = number of moles of base (nB)

nB = MB X VB

MB = molarity of base , VB = volume of base

Hence nA = MB X VB = 0.199 X 16.11mL

nA = 3.20 mmoles = 3.20 X 10-3 moles

number of moles of acid (nA ) = given mass of acid (WA ) / Molar mass of acid (MA)

Hence WA / MA = 3.20 X 10-3 moles, Because

MA = WA / 3.20 X 10-3 moles = 1.263g / 3.20 X 10-3 = 394.68 g/mol

Q2. Sodium hydroxide needs to be standardised because it absorbs and reacts with atmospheric gases.

NOTE: Please send other questions separately.

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