The concentration of a chemical solution refers to the amount of solute that is dissolved in a solvent. We normally think of a solute as a solid that is added to a solvent (e.g., adding table salt to water), but the solute could just as easily exist in another phase. For example, if we add a small amount of ethanol to water, then the ethanol is the solute and the water is the solvent. If we add a smaller amount of water to a larger amount of ethanol, then the water could be the solute!
Units of Concentration
Once you have identified the solute and solvent in a solution, you are ready to determine its concentration. Concentration may be expressed several different ways, using percent composition by mass, volume percent, mole fraction, molarity, molality, or normality.
1. Percent Composition by Mass (%) This is the mass of the solute divided by the mass of the solution (mass of solute plus mass of solvent), multiplied by 100.
Example:
Determine the percent composition by mass of a 100 g salt solution which contains 20 g salt.
Solution:
20 g NaCl / 100 g solution x 100 = 20% NaCl solution
2. Volume Percent (% v/v)
Volume percent or volume/volume percent most often is used when preparing solutions of liquids. Volume percent is defined as:
v/v % = [(volume of solute)/(volume of solution)] x 100%
Note that volume percent is relative to volume of solution, not volume of solvent. For example, wine is about 12% v/v ethanol. This means there are 12 ml ethanol for every 100 ml of wine. It is important to realize liqud and gas volumes are not necessarily additive. If you mix 12 ml of ethanol and 100 ml of wine, you will get less than 112 ml of solution.
As another example. 70% v/v rubbing alcohol may be prepared by taking 700 ml of isopropyl alcohol and adding sufficient water to obtain 1000 ml of solution (which will not be 300 ml).
3. Mole Fraction (X)
This is the number of moles of a compound divided by the total number of moles of all chemical species in the solution. Keep in mind, the sum of all mole fractions in a solution always equals 1.
Example:
What are the mole fractions of the components of the solution formed when 92 g glycerol is mixed with 90 g water? (molecular weight water = 18; molecular weight of glycerol = 92)
Solution:
90 g water = 90 g x 1 mol / 18 g = 5 mol water
92 g glycerol = 92 g x 1 mol / 92 g = 1 mol glycerol
total mol = 5 + 1 = 6 mol
xwater = 5 mol / 6 mol = 0.833
x glycerol = 1 mol / 6 mol = 0.167
It's a good idea to check your math by making sure the mole fractions add up to 1:
xwater + xglycerol = .833 + 0.167 = 1.000
4. Molarity (M)
Molarity is probably the most commonly used unit of concentration. It is the number of moles of solute per liter of solution (not necessarily the same as the volume of solvent!).
Example:
What is the molarity of a solution made when water is added to 11 g CaCl2 to make 100 mL of solution?
Solution:
11 g CaCl2 / (110 g CaCl2 / mol CaCl2) = 0.10 mol CaCl2
100 mL x 1 L / 1000 mL = 0.10 L
molarity = 0.10 mol / 0.10 L
molarity = 1.0 M
5. Molality (m)
Molality is the number of moles of solute per kilogram of solvent. Because the density of water at 25°C is about 1 kilogram per liter, molality is approximately equal to molarity for dilute aqueous solutions at this temperature. This is a useful approximation, but remember that it is only an approximation and doesn't apply when the solution is at a different temperature, isn't dilute, or uses a solvent other than water.
Example:
What is the molality of a solution of 10 g NaOH in 500 g water?
Solution:
10 g NaOH / (4 g NaOH / 1 mol NaOH) = 0.25 mol NaOH
500 g water x 1 kg / 1000 g = 0.50 kg water
molality = 0.25 mol / 0.50 kg
molality = 0.05 M / kg
molality = 0.50 m
6. Normality (N)
Normality is equal to the gram equivalent weight of a solute per liter of solution. A gram equivalent weight or equivalent is a measure of the reactive capcity of a given molecule. Normality is the only concentration unit that is reaction dependent.
Example:
1 M sulfuric acid (H2SO4) is 2 N for acid-base reactions because each mole of sulfuric acid provides 2 moles of H+ ions. On the other hand, 1 M sulfuric acid is 1 N for sulfate precipitation, since 1 mole of sulfuric acid provides 1 mole of sulfate ions.
Dilutions
You dilute a solution whenever you add solvent to a solution. Adding solvent results in a solution of lower concentration. You can calculate the concentration of a solution following a dilution by applying this equation:
MiVi = MfVf
where M is molarity, V is volume, and the subscripts i and f refer to the initial and final values.
Example:
How many millilieters of 5.5 M NaOH are needed to prepare 300 mL of 1.2 M NaOH?
Solution:
5.5 M x V1 = 1.2 M x 0.3 L
V1 = 1.2 M x 0.3 L / 5.5 M
V1 = 0.065 L
V1 = 65 mL
So, to prepare the 1.2 M NaOH solution, you pour 65 mL of 5.5 M NaOH into your container and add water to get 300 mL final volume.
Monday, November 29, 2010
How Carbon Monoxide Detectors Work
Carbon monoxide is an invisible odorless gas that is the leading cause of accidental poisoning deaths in America. Cabon Monoxide Detectors can alert you to dangerous levels of carbon monoxide.
Originally, carbon monoxide detectors were simple opto-chemical detectors that indicated the presence of carbon monoxide by exhibiting a color change when carbon monoxide reacted with a chemical on a white pad, producing a brownish or black color. These detectors do not require an external power source to function, but modern designs use audible alarms to confer a higher level of protection:
How the First Carbon Monoxide Detectors Worked
Biomimetic Carbon Monoxide Sensors
A opto-chemical or gel sensor interacts with synthetic hemoglobin, darkening in color when carbon monoxide is present and lightening in color when carbon monoxide concentrations are low. A light sensor may be used to detect the change in light levels to sound an alarm.Semiconductor Carbon Monoxide Detectors
An integrated circuit monitors a sensor, tripping the alarm when concentrations of carbon monoxide are high. The sensor is made from thin wires of semiconducting tin dioxide that rest on an insulating ceramic base. Increasing carbon monoxide concentration reduces the electrical resistance of the sensor, causing the alarm to sound.Electrochemical Carbon Monoxide Detectors
This is an electrochemical cell that is designed to produce current in relation to the amount of carbon monoxide present in the air. Carbon monoxide is oxidized to carbon dioxide at one electrode while oxygen is consumed at the other electrode. Sulfuric acid is the usual electrolyte that separates the electrodes. The current triggers the alarm or can even be used to quantify the amount of carbon monoxide that is present.Chemical Oxygen Demand (COD)
Basic:
Oxidation of organic substances with potassium dichromate and silver sulfate excess in boiling sulfuric acid. The amount of potassium dichromate is not reduced during the oxidation reaction is determined by how titrimetrik with standard solution of ferrous ammonium sulfate (FAS) and indicators feroin. Chloride ion concentration above 1 mg / L is closed with mercury sulfate.
Reaction:
CxHyOz + Cr2O72-→ CO2 + H2O + Cr3+
Cr2O72-(excess) + Fe2 + → Fe3 + + 2Cr3+ + H2O
Reagents:
1. Examples
2. Sulfuric acid 98%
3. Silver sulfate solution
4. Mercury sulfate solution
5. Potassium dichromate solution
6. Ferrous ammonium sulfate solution
7. Solution of potassium hydrogen phthalate
8. Feroin indicator solution
Tools:
1. Pumpkin peck 50 ml, 100 ml, and 1000 ml
2. Goblets 250, 500, and 1000 ml
3. 25.50 measuring cup, and 100 ml
4. Watch glass
5. 25 ml Burette + clamp + Standards
6. The temperature of the appliance COD (cod tube, air condensers, equipment destruction, ice water bath, and shelves)
7. Electronic Scales 4 decimal places (Mettler AE 260)
8. Magnetic stir bar magnet and 2.5 cm
How it works:
1. In the pumpkin destruction COD included consecutive 20 ml sample or have been diluted, 5 ml of mercury sulfate. 10 ml of 0.1 N potassium dichromate and boiling stones
2. Air condenser mounted above the pumpkin COD then added 40 ml 98% sulfuric acid through the upper air condenser carefully
3. Destruction tool heated to the perfect red dots mark
4. Examples that have been prepared (no. 2) is inserted into the device destruction and didestruksikan for 120 minutes
5. When didestruksi / reflux lasted 10 minutes add 5 ml of silver sulphate through the upper condenser
6. Examples that have been didestruksi cooled in the open air to room temperature
7. Added 25 ml of distilled water through the upper air condenser as a rinse
8. Air condenser removed and cooled in ice water sample
9. Example titrated with 0.1 N ferrous ammonium sulfate with feroin indicator until the end point (color change from blue green to red brown)
10. Blank determination made by the same treatment as samples, but samples of distilled water replaced
11. Done setting the standard solution of potassium hydrogen phthalate with the same treatment as an example, but an example is replaced with a standard solution of potassium hydrogen phthalate
12. Added 10 ml of 0.1 N potassium dichromate solution into a flask COD sample / standard / blank that has been titrated to end point. Then titrated back with a solution of 0.1 N ferrous ammonium sulfate until the endpoint (this step for the determination of FAS solution of normality 0.1 N)
Calculation:
1. The calculation of the concentration of ferrous ammonium sulfate (FAS)
FAS = ml K2Cr2O7 N x N K2Cr2O7
ml Fas
1. COD value calculation COD (mg O2 / L) = (a-b) x c x 8000 x d
v
Description:
a = volume of FAS to titrate the blank. ml
b = volume umtuk FAS titration sample.
c = normality of FAS. N
d = dilution factor
v = volume of sample. ml
Oxidation of organic substances with potassium dichromate and silver sulfate excess in boiling sulfuric acid. The amount of potassium dichromate is not reduced during the oxidation reaction is determined by how titrimetrik with standard solution of ferrous ammonium sulfate (FAS) and indicators feroin. Chloride ion concentration above 1 mg / L is closed with mercury sulfate.
Reaction:
CxHyOz + Cr2O72-→ CO2 + H2O + Cr3+
Cr2O72-(excess) + Fe2 + → Fe3 + + 2Cr3+ + H2O
Reagents:
1. Examples
2. Sulfuric acid 98%
3. Silver sulfate solution
4. Mercury sulfate solution
5. Potassium dichromate solution
6. Ferrous ammonium sulfate solution
7. Solution of potassium hydrogen phthalate
8. Feroin indicator solution
Tools:
1. Pumpkin peck 50 ml, 100 ml, and 1000 ml
2. Goblets 250, 500, and 1000 ml
3. 25.50 measuring cup, and 100 ml
4. Watch glass
5. 25 ml Burette + clamp + Standards
6. The temperature of the appliance COD (cod tube, air condensers, equipment destruction, ice water bath, and shelves)
7. Electronic Scales 4 decimal places (Mettler AE 260)
8. Magnetic stir bar magnet and 2.5 cm
How it works:
1. In the pumpkin destruction COD included consecutive 20 ml sample or have been diluted, 5 ml of mercury sulfate. 10 ml of 0.1 N potassium dichromate and boiling stones
2. Air condenser mounted above the pumpkin COD then added 40 ml 98% sulfuric acid through the upper air condenser carefully
3. Destruction tool heated to the perfect red dots mark
4. Examples that have been prepared (no. 2) is inserted into the device destruction and didestruksikan for 120 minutes
5. When didestruksi / reflux lasted 10 minutes add 5 ml of silver sulphate through the upper condenser
6. Examples that have been didestruksi cooled in the open air to room temperature
7. Added 25 ml of distilled water through the upper air condenser as a rinse
8. Air condenser removed and cooled in ice water sample
9. Example titrated with 0.1 N ferrous ammonium sulfate with feroin indicator until the end point (color change from blue green to red brown)
10. Blank determination made by the same treatment as samples, but samples of distilled water replaced
11. Done setting the standard solution of potassium hydrogen phthalate with the same treatment as an example, but an example is replaced with a standard solution of potassium hydrogen phthalate
12. Added 10 ml of 0.1 N potassium dichromate solution into a flask COD sample / standard / blank that has been titrated to end point. Then titrated back with a solution of 0.1 N ferrous ammonium sulfate until the endpoint (this step for the determination of FAS solution of normality 0.1 N)
Calculation:
1. The calculation of the concentration of ferrous ammonium sulfate (FAS)
FAS = ml K2Cr2O7 N x N K2Cr2O7
ml Fas
1. COD value calculation COD (mg O2 / L) = (a-b) x c x 8000 x d
v
Description:
a = volume of FAS to titrate the blank. ml
b = volume umtuk FAS titration sample.
c = normality of FAS. N
d = dilution factor
v = volume of sample. ml
Sunday, November 28, 2010
Titration
Titration there are times when people refer to as the volumetric method, this is due to the volume of solution in the titration measurements play an important role. From taking a particular analyte by the volume until the reading of the volume of titrant used for titration's end affect all the results of the analysis. Therefore, the proper use of equipment in the titration also should not be overlooked.Volumetry method differentiated the types of reactions involved between titrant and analyte are:Acid-Base. There are a lot of acid and alkaline compounds which can be determined by titration. Both strong acid or strong base, the end point titrasipun very easily observed with the use of acid-base indicators such as fenolphtalein (PP), red metal, metallic orange, and others. At the equivalent point obtained, the solution is neutral but with a little addition of titrant to reach the end point of titration is enough to change the color of acid-base indicator. Another way is to use pHmeter. Weak acid and weak base can also be titrated as well as the organic acid is titrated with non-water solvent.Reduction-Oxidation. Substances that are oxidizing agents such as KMnO4, K2CrO4, I2, and substances that are reducing agents such as H2C2O4, Fe2, Sn2 can be determined by this titration method. Redox reactions involved as titrant and analyte react. Some of the redox titration method does not require the indicator to look like a titration end point titration of KMnO4 and H2C2O4 caused KMnO4 itself is colored. Amylum usually used for titrations involving I2.Kompleksometri. Complex formation reaction between EDTA and metal ions underlies this method. EDTA is a widely used type of titrant for titration kompleksometri and react with many metals, reaksinyapun can be controlled by controlling the pH of the solution.Precipitation. Reaction formation of sediment form the basis of this method. Titrant and analyte react to form a precipitate, such as the determination of chloride ion using AgNO3 titrant. Indicators can be used to determine the end point titration for example K2CrO4 for titration using silver nitrate titrant.
Subscribe to:
Comments (Atom)