Lab 4: Weak Acid Unknown
When testing the acid, use only between 0.2 g and 0.3 g for each trial (get as precise a measurement as you can). The general procedure is to weigh out your acid, dissolve it in water, add a couple drops of the indicator (phenolphthalein), and then add the sodium hydroxide until you note a color change (from clear to pink). When the color change occurs, you have added enough base to completely react with the acid (the endpoint). You are allowed three trials, and will be graded on accuracy.
HA(aq) + NaOH(aq) NaA(aq) + H2O
Trial 1: .1010 M of NaOH = moles of NaOH / 0.0299 L = 0.0030199 mols of NaOH
Trial 2: .1010 M of NaOH = moles of NaOH / 0.0227 L = 0.0022927 mols of NaOH
Trial 3: .1010 M of NaOH = moles of NaOH / 0.0158 L = 0.0015958 mols of NaOH
After we find the moles of NaOH, we can use that to find the molar mass of each acid.
Trial 1: 0.261g / 0.0030199 = 86.43 mm of acid
Trial 2: 0.233g / 0.0022927 = 101.63 mm of acid
Trial 3: 0.254g / 0.0015958 = 159.17 mm of acid
To find the average molar mass, we just add up the three molar masses we calculated and divide them by three, 86.43 + 101.63 + 159.17 / 3 = 115.74 avg mm
3. Original equation: HA + NaOH NaA + H2O Diprotic equation: H2A + 2NaOH Na2A + 2H2O Old mole ratio: 1:1 New mole ratio: 1:2 Example: since the ratio is 1 to 2, the mole ratio is going to decrease by half, the molar mass of the acid is going to increase by half.
4. If someone were to add NaCl to my solid unknown acid the molar mass would be affected by being higher than the molar mass of the actual value we got in lab. We would get the mass of what the acid was as well as the mass of the table salt, therefore, producing a larger number than expected.