Example Abstracts for a General Chemistry Lab

Title

Gravimetric Determination of the Solubility Product Constant for Lead (II) Chloride, PbCl₂

Introduction

In this experiment, the equilibrium exhibited by slightly soluble ionic compounds in water is explored. Most ionic compounds, even those called "soluble", have a limited solubility in water. If more than this amount is added, some solid will remain undissolved. In a saturated solution at a particular temperature, equilibrium exists between the undissolved and dissolved solid. Slightly soluble ionic compounds are often called "insoluble" because they have a relatively low solubility (little dissolves before equilibrium is reached). Lead (II) chloride, the insoluble ionic compound used, is assumed to dissociate according to equation 1.

( 1 )

PbCl₂(s)

Pb²⁺(aq) + 2 Cl⁻(aq)

K_sp,

the equilibrium constant for the dissociation reaction, is written according to equation 2.

( 2 )

K_sp = [Pb²⁺][Cl⁻]²

Mass measurements are made in order to determine the amounts of dissociated and undissociated PbCl₂.

K_sp

is then calculated using Eq. 2. Since PbCl₂ is "insoluble",

K_sp

should be very small (<<1). This reflects the fact that the concentration of the dissolved ions, Pb²⁺ and Cl^–, is very low.

Abstract

Instructions

Rate the following abstracts from 1 to 5.

1 = beginning, 2 = developing, 3 = adequate, 4 = accomplished, 5 = exemplary

Sample Abstracts

The

K_sp

for PbCl₂ dissociation was found. Three trials were performed using about 0.770 g PbCl₂ each time. One trial was performed in 25.00 mL pure water; one trial was performed in 25.00 mL 0.10 M NaCl; and, one trial was performed in 25.00 mL 0.10 M Pb(NO₃)₂ so the effect of additional dissolved ions could be assessed.

K_sp

of PbCl₂ was found to be 1.59 × 10^–5. Even though it was hard to measure the Pb²⁺ and Cl^– concentrations, the results were pretty good.

Equilibrium dissociation constants that compare favorably with literature values can be obtained by the gravimetric method used in this work. The solubility product constant,

K_sp,

of lead (II) chloride was found to be 1.59 × 10^–5 ± 6.00 × 10^–7 at 298 K, which is within 1% of the accepted value. Primary sources of error can be minimized if the work is performed carefully.

We calculated

K_sp

for PbCl₂ which is an ionic compound that doesn't dissolve in water too much but does a little bit depending on factors like temperature and other things. We had to do three tests with solid PbCl₂ and pure water or 0.10 M NaCl or 0.10 M

Pb(NO₃)₂

and then figure out how much Pb²⁺ and Cl^– were in the solution part. We got a

K_sp

that was close to the value our TA said was right.

PbCl₂, an "insoluble" ionic compound, has a low solubility product constant (

K_sp

) of 1.6 × 10^–5 at 25°C. Using gravimetric analysis, the experimentally determined

K_sp

of PbCl₂ was found to be 1.59 × 10^–5 ± 6.00 × 10^–7. The small percent difference between the expected and observed

K_sp

values indicates that this method of analysis is a valid and accurate way of determining the extent of dissociation of slightly soluble ionic compounds in water. Problems such as precipitate loss and/or contamination during filtration can introduce error if care is not taken during the experiments.