Answer to Question #257949 in General Chemistry for Ayesha

Question

We have to find the amount of chromium in grams by doing gravimetric analysis. Our sample solution is potassium dichromate. The salt we've choosed is lead acetate.. kindly guide me how much amount of lead acetate should I take as well as what amount (ml )of potassium dichromate should I use..

Answer 1

Deposition methods are the most common gravimetric analysis methods. A weighed portion of the analyte is dissolved in water or another solvent, and the analyte is precipitated with a reagent in the form of a poorly soluble compound. The resulting precipitate is filtered off, washed, dried, calcined and weighed. By the mass of the precipitate after calcination, the mass fraction of the determined element in the sample is calculated.

Since the precipitated substance may not correspond to what is obtained after calcination, the precipitated and gravimetric (weight) forms of the precipitate are distinguished. The precipitated form is a compound that precipitates from solution upon interaction with the appropriate reagent during the analysis, and the gravimetric form is formed from precipitated upon drying or calcination.

2Pb(CH₃COO)₂ + K₂Cr₂O₇ + H₂O → 2PbCrO₄ + 2KCH₃COO + 2CH₃COOH

Lead cations form with chromate ions СrO4^2– and dichromate ions Сr2O7^2– in an acetic acid medium yellow crystalline precipitate of lead chromate РbСrO4:

Рb²⁺ + СrO4 ^2– → РbСrO4↓

2Рb²⁺ + Сr2O7 ^2– + 2СН3СОО^– + H2O → 2РbСrO4 + 2СН3СООН.

In this case, the deposited form is yellow crystalline precipitate of lead chromate РbСrO4.

To carry out the reaction, you need to take 100 ml of lead acetate and 150 ml of potassium dichromate.