Precipitation Reactions Janelle finds a recipe for the preparation of the pigment chrome yellow. She dissolves lead(II) nitrate and potassium chromate in separate volumes of water, and then mixes the two solutions. She obtains a yellow precipitate. Identify the precipitate and write a balanced equation for the reaction she carried out.

Question

Precipitation Reactions

Janelle finds a recipe for the preparation of the pigment chrome yellow. She dissolves lead(II) nitrate and potassium chromate in separate volumes of water, and then mixes the two solutions. She obtains a yellow precipitate. Identify the precipitate and write a balanced equation for the reaction she carried out.

Accepted Answer

Predicting products and balancing their formulas: The starting solutions contain Pb²⁺(aq), NO^3–(aq), K⁺ (aq), and CrO₄^2–(aq). We can write the possible product formulas (without physical state symbols for now) by exchanging cations and anions to get the other possible arrangement. The product formulas must be written so that they are balanced with the cation and anion charges cancelling to give a neutral overall charge or zero. In the i rst product written, the lead ion and chromate are 2+ and 2–, respectively, so the formula is balanced as is, PbCrO₄.

In the second product written, the potassium ion and nitrate ion are 1+ and 1–, respectively, so that formula is also already balanced, KNO₃. In this case, no subscripts are needed to balance the formulas.

Pb(NO₃)₂(aq) + K₂CrO₄(aq) → PbCrO₄ + KNO₃

Identifying the precipitate: Because Janelle obtained a precipitate, we know one product must be an insoluble solid. The solubility rules in Table 5.3 say that most compounds of potassium are soluble and all nitrates are soluble, so KNO₃ is water soluble. The only other product is PbCrO4, so it must be the insoluble solid and the identity of the yellow precipitate. We use physical state symbols to show that the precipitate PbCrO4(s) is an insoluble solid and that KNO₃(aq) is soluble in water:

Pb(NO₃)₂(aq) + K₂CrO₄(aq) → PbCrO₄(s) + KNO₃(aq)

Changing coefficients to balance the equation: Note that potassium and nitrate ions are not balanced. Changing the coefficient in front of KNO₃(aq) from 1 to 2 balances the equation:

Pb(NO₃)₂(aq) + K₂CrO₄(aq) → PbCrO₄(s) + 2KNO₃(aq)

Question 5.10: Precipitation Reactions Janelle finds a recipe for the prepa...

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