If a 2.6 g of weak diprotic acid were dissolved in 100 mL of water and a 10 mL aliquot of this solution required 21.6 mL of 0.1 M NaOH to reach the first endpoint, what are the equivalent and formula weights of H2A?
I think I first need to find the moles of NaOH. Then, by using the formula.
H2A + 2NaOH ----> Na2A + 2 H20
I am stuck from here. Do I find the equivalence point of each and add them?
This topic's pretty old, but I figured it would be worth it to have the new way to solve the problem posted as well. If you're like me you're a young whippersnapper who's never even heard of normality.
It's actually not as hard as DrBob says.
Molarity = moles/liter, so convert your NaOH added to liters then plug everything in like so:
0.1 M = moles/0.0216 L
This will give you the moles of NaOH in the solution. From the balanced equation you know there are 2 moles NaOH per 1 mol of H2A, so dividing moles of NaOH by 2 will give you moles of H2A.
The formula weight is simply the grams of H2A in the solution (0.26g) divided by the moles of H2A you just calculated. Divide this answer by 2 to find the equivalent weight (because H2A has 2 equivalents, or H+ ions).
To find the equivalent and formula weights of H2A, you can follow these steps:
Step 1: Calculate the moles of NaOH used.
Moles of NaOH = volume of NaOH solution (L) x molarity of NaOH (mol/L)
= 21.6 mL ÷ 1000 mL/L x 0.1 mol/L
= 0.00216 moles
Step 2: Determine the mole ratio between NaOH and acid, based on the balanced chemical equation.
From the balanced equation, you can see that 2 moles of NaOH react with 1 mole of H2A. So, every 2 moles of NaOH used is equivalent to 1 mole of H2A.
Step 3: Calculate the moles of H2A.
Moles of H2A = (moles of NaOH used) ÷ (mole ratio)
= 0.00216 moles ÷ 2
= 0.00108 moles
Step 4: Calculate the mass of H2A.
Mass of H2A = moles of H2A x molar mass of H2A
= 0.00108 moles x molar mass (g/mol)
Step 5: Calculate the equivalent weight of H2A.
Equivalent weight of H2A = mass of H2A ÷ moles of H2A
= molar mass (g/mol) ÷ moles of H2A
Now, to find the molar mass of H2A, we need to use the additional information given in the problem:
The original solution had 2.6 grams of H2A dissolved in 100 mL of water. We can assume that the density of the solution is 1 g/mL, which means the volume of the solution is equivalent to its mass.
Step 6: Calculate the molar mass of H2A.
Molar mass of H2A = mass of H2A ÷ volume of solution (L)
= 2.6 g ÷ 1000 mL/L
= 0.0026 mol
Finally, the equivalent weight of H2A can be calculated using the molar mass of H2A from step 6 and the moles of H2A from step 3:
Equivalent weight of H2A = molar mass (g/mol) ÷ moles of H2A
I hope this helps you calculate the equivalent and formula weights of H2A!
To find the equivalent and formula weights of H2A, we need to use the given information. Let's break down the steps:
Step 1: Find the moles of NaOH used
To find the moles of NaOH used, we can use the given concentration and volume of NaOH used.
Moles of NaOH = Concentration of NaOH × Volume of NaOH used in liters
= 0.1 M × (21.6 mL ÷ 1000 mL/L)
= 0.00216 moles of NaOH
Step 2: Determine the moles of H2A reacted
By examining the balanced equation, we can see that 2 moles of NaOH react with 1 mole of H2A. Therefore, the moles of H2A reacted would be half the moles of NaOH used.
Moles of H2A reacted = 0.00216 moles of NaOH ÷ 2
= 0.00108 moles of H2A
Step 3: Calculate the equivalent weight of H2A
The equivalent weight is the weight of a substance that supplies or reacts with one equivalent of a given species. In this case, since H2A is a diprotic acid, it will have two equivalents per mole.
Equivalent weight of H2A = Molecular weight of H2A ÷ Number of equivalents
= (Mass of H2A ÷ Moles of H2A) ÷ Number of equivalents
= (2.6 g ÷ 0.00108 moles) ÷ 2
≈ 1203 g/equivalent
Step 4: Calculate the formula weight of H2A
The formula weight is the weight of one formula unit of a substance.
Formula weight of H2A = Molecular weight of H2A = 2 × Atomic weight of Hydrogen + Atomic weight of Carbon + 4 × Atomic weight of Oxygen
To determine the atomic weights, you can refer to the periodic table. By looking up the atomic weights for hydrogen (H), carbon (C), and oxygen (O), you can calculate the formula weight.
Step 5: Finalize the answer
By completing the calculations from Step 4, you can determine the formula weight of H2A.
It's essential to follow these steps to ensure you accurately solve the problem and find the correctly weighted values for H2A.
The easy way to do this is
mL x N x millieq wt = grams.
You had 2.6g in 100 and took a 10 mL aliquot to titrate; therefore, the mass of the sample titrated was 0.26g
So 21.6 x 0.1 x mew = 0.26.
Solve for mew. Multiply by 1000 to find equivalent weight and multiply by 2 to find the formula mass. I am so sorry that texts/IUPAC and others have done away with normality. One step solved this problem. It would have taken half a page and multiple steps to do it the "new" way.