You are presented with a mystery as part of your practical experiment. You have a solution of Pb(NO3)2 that has a worn label making it impossible to read. You know the concentration is below 1.0 M as you can make out "0.xxx" at the beginning of the label. In order to determine the concentration, you decide to precipitate out the lead in the solution as PbSO4.
If you added 1.0 mL of the unknown Pb(NO3)2 to a test tube, what is the amount of H2SO4 in mL you will need to add to be sure the H2SO4 is the excess reagent?

For this reaction, we know that the max concentration of Pb(NO₃) according to the bottle is 0.999M and to ensure the other reactant in the reaction is in excess, we'll do the calculation with a Pb(NO₃) that's a bit higher, that is, 1.0M.

Knowing that Concentration in mol/L = (number of moles)/(volume in L)

Number of moles of Pb(NO₃) added = concentration in mol/L × volume in L = 1 × 0.001 = 0.001 mole

According to the reaction,

1 mole of Pb(NO₃) reacts with 1 mole of H₂SO₄

0.001 mole of Pb(NO₃) will react with 0.001×1/1 mole of H₂SO₄

Therefore number of H₂SO₄ required for the reaction and for the H₂SO₄ to be in excess is 0.001 mole of H₂SO₄

So, the concentration of commercial H₂SO₄ is usually 18.0M, using this as the assumed value.

Volume of H₂SO₄ = (number of H₂SO₄ required for it to be in excess)/(concentration of H₂SO₄)

Volume of H₂SO₄ = 0.001/18 = 0.0000556 L = 0.0556 mL.

QED!!!

mickymike92 mickymike92 · Answer 2 · 2022-02-03T14:57:34+01:00

Assuming concentrated H₂SO₄ is used for the precipitation, the minimum volume of concentrated H₂SO₄ required to be the excess reagent is 0.0556 mL.

What volume of H₂SO₄ is required for H₂SO₄ to be in excess

The equation of the reaction between H₂SO₄ and Pb(NO₃)₂ is given below:

Pb(NO₃)₂ + H₂SO₄ -----> PbSO₄ + 2HNO₃

Assuming that the concentration of Pb(NO₃)₂ is 1.0M.

Number of moles of Pb(NO₃)₂ in 1.0 mL solution is calculated as follows:

Number of moles = molarity * volume in L

Volume of Pb(NO₃)₂ = 1.0 mL = 0.001 L

Number of moles of Pb(NO₃)₂ = 1.0 * 0.001

Number of moles of Pb(NO₃)₂ = 0.001 moles

From the equation of reaction:

1 mole of Pb(NO₃)₂ reacts with 1 mole of H₂SO₄

0.001 mole of Pb(NO₃)₂ will react with 0.001 moles of H₂SO₄

Therefore number of moles H₂SO₄ required for the reaction and for the H₂SO₄ to be in excess is 0.001 mole of H₂SO₄

Assuming the H₂SO₄ required is concentrated H₂SO₄:

Concentration of concentrated H₂SO₄ = 18.0M

The volume of concentrated H₂SO₄ required is calculated as follows:

Volume = moles/molarity

Volume of H₂SO₄ = 0.001/18

Volume of H₂SO₄ = 0.0000556 L

Volume of concentrated H₂SO₄ required = 0.0556 mL.

Therefore, the minimum volume of concentrated H₂SO₄ required to be the excess reagent is 0.0556 mL.

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