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Molarity

The most common unit of solution concentration is

molarity (M) The molarity of a solution is defined as the number of moles of solute per one liter of solution. Note that the unit of volume for molarity is liters, not milliliters or some other unit. Also note that one liter of solution contains both the solute and the solvent. Molarity, therefore, is a ratio between moles of solute and liters of solution. To prepare laboratory solutions, usually a given volume and molarity are required. To determine molarity, the formula weight or molar mass of the solute is needed. The following exam ples illustrate the calculations for preparing solutions. If starting with a solid, use the following procedure: mass, MM s s) required using equation 1. eq. 1. g s = MM s x M x NaCl g NaCl xx g NaCl

If starting with a solution or liquid reagent:

2 2 ) the final solution should be.

Basic Concepts of Preparing Solutions

Many of the reagents used in science are in the form of solutions which need to be purchased or prepared. For many purposes, the exact value of concentration is not critical; in other cases, the concentration of the solution and its method of preparation must be as accurate as is designed for both the novice and experienced solution maker. It provides valuable information on the basic concepts of preparing solu tions and instructions for preparing most solutions required in the high when high quality and fresh chemicals and solvents are used, and meticulous procedures are followed. Many of the solutions described

TOSAVE

valuable laboratory prep time.

The section is divided into several parts for your convenience. Basic concepts of preparing solutions

1 ) of starting, more concentrated solu tion. 1 ) required using equa 1 2 1 1 = M 2 2 M 1 1 = M 2 2 1 1

Mass percent solutions

are defined based on the grams of solute by mass solution.

Volume percent solutions

are defined as milliliters of solute per 2

MAKING

APPROX

M, OF

SOLUTION

IS A BY

VOLUME

solution.

Mass-volume percent solutions

are also very common. These

© 2011 Flinn Scientific, Inc. All Rights Reserved.BASIC CONCEPTS OF PREPARING SOLUTIONS continued on next page.

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You may wish to convert mass percent to volume percent or vice versa. If so, follow this procedure: 1. The formula for determining the volume of the component mass of ethyl alcohol density of ethyl alcohol mass of the solution by the density of the solution. the component by the volume of the solution. mass density handbook) Reverse the procedure to convert volume percent to mass percent. * The volume percent statement generally is accurate but the volume percen t is not always calculated directly from the volumes of the mixed ingredients bec ause the final volume may not equal the sum of the separate volumes. In our solut ion (No.

2 above) note that if the alcohol volume (12.6 mL) is added to the wa

ter volume (90 mL), the final volume is less than 102.6 mL.

Basic Concepts of Preparing Solutionscontinued

To determine the molarity of a mass percent solution, the density of the solution is required. Use the following proce dure: the solution by the density of the solution. mass = volume x density solution.

Change to the decimal equivalent.

the whole solution. the solution to find the molarity of the solution. x x x = MM HCl Buffer: A solution which tends to maintain a constant pH when excess acid or base is added.

Concentrated:

For some commonly used acids and bases, the

or as a pure liquid.

Concentration:

The relative amount of solute and solvent in a

solution.

Hydrates:

water molecules into account. Miscible: The ability of two liquids to be completely soluble in one another.

Molality:

moles of solute divided by the number of kilograms of solvent.

Molar Mass:

The mass of a mole of any element or compound.

Molarity:

moles of solute divided by liters of solution. "Your Safer Source for Science Supplies"

Name / Formula / F.W. Concentration g/L

Aluminum chloride

3 2

Aluminum nitrate

3 3 2 O

Aluminum sulfate

Al 2 4 3 2 O

Ammonium acetate

NH 4 2 H 3 O 2

Ammonium chloride

NH 4

Ammonium nitrate

NH 4 NO3

Ammonium sulfate

4 2 4

Barium chloride

2 2 O

Barium hydroxide

2 2 O

Barium nitrate

3 2

Bismuth nitrate

3 3 2 3

Preparation of Simple Inorganic Salt Solutions

PREPARATION OF SIMPLE INORGANIC SALT SOLUTIONS

continued on next page.

Name / Formula / F.W. Concentration g/L

Bismuth trichloride

3

Cadmium chloride

2 1 2H 2 O

Cadmium nitrate

3 2 2 O

Calcium acetate

2 H 3 O 2 2 2

Calcium chloride

2 2

Calcium hydroxide

2

Calcium nitrate

3 2 2

Chromium(III) chloride

3 2 O

Chromium(III) nitrate

3 3 2 O

Cobalt(II) chloride

2 2 O

Cobalt(II) nitrate

3 2 2 O

Copper(II) chloride

2 2

Copper(II) nitrate

3 2 2

Copper(II) sulfate

4 2

Iron(II) sulfate

4 2 2 4

Iron(III) chloride

3 2

Iron(III) nitrate

3 3 2 O Add solid to acid solution, stir, then add to water. Dilute to 1 L. Remember, always add acid to water. Approximate amount for 1 L of saturated solution. Keep adding solute unt il it no longer dissolves; stir for 1 hour, then filter.

Normality:

2 4 2 4

Saturated Solution:

A solution that contains the maxi

mum amount of a particular solute that will dissolve at that temperature. Solute: The substance which is dissolved, or has gone into solution Solution: A uniform homogeneous mixture of two or more substances. The individual substances may be present in varying amounts.

Solvent:

solution.

Standard Solution:

significant figures, used in quantitative analysis or an analytical procedure. Supersaturated Solution: A solution that contains more solute than equilibrium conditions allow; it is unstable and the solute may precipitate upon slight agitation or addition of a single crystal.

Preparation of Simple Inorganic Salt Solutions

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1-800-452-1261

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Safety Reference

1. 3.

Transfer

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