How pH and pKa Are Related?

pH vs pKa

pH is a measure of the concentration of hydrogen ions in an aqueous solution. pKa (acid dissociation constant) is related, but more specific, in that it helps you predict what a molecule will do at a specific pH. Essentially, pKa tells you what the pH needs to be in order for a chemical species to donate or accept a proton.

• The lower the pH, the higher the concentration of hydrogen ions, [H+]. The lower the pKa, the stronger the acid and the greater its ability to donate protons.
• pH depends on the concentration of the solution. This is important because it means a weak acid could actually have a lower pH than a diluted strong acid. For example, concentrated vinegar (acetic acid, which is a weak acid) could have a lower pH than a dilute solution of hydrochloric acid (a strong acid). On the other hand, the pKa value is a constant for each type of molecule. It is unaffected by concentration.
• Even a chemical ordinarily considered a base can have a pKa value because the terms "acids" and "bases" simply refer to whether a species will give up protons (acid) or remove them (base). For example, if you have a base Y with a pKa of 13, it will accept protons and form YH, but when the pH exceeds 13, YH will be deprotonated and become Y. Because Y removes protons at a pH greater than the pH of neutral water (7), it is considered a base.

Relating pH and pKa With the Henderson-Hasselbalch Equation

If you know either pH or pKa you can solve for the other value using an approximation called the Henderson-Hasselbalch equation:

pH is the sum of the pKa value and the log of the concentration of the conjugate base divided by the concentration of the weak acid.

At half the equivalence point:

pH = pKa

It's worth noting sometimes this equation is written for the Ka value rather than pKa, so you should know the relationship:

pKa = -logKa

Assumptions That Are Made for the Henderson-Hasselbalch Equation

The reason the Henderson-Hasselbalch equation is an approximation is because it takes water chemistry out of the equation. This works when water is the solvent and is present in a very large proportion to the [H+] and acid/conjugate base. You shouldn't try to apply the approximation for concentrated solutions. Use the approximation only when the following conditions are met:

• -1 < log ([A-]/[HA]) < 1
• Molarity of buffers should be 100x greater than that of the acid ionization constant Ka.
• Only use strong acids or strong bases if the pKa values fall between 5 and 9. Continue reading..

What causes Hospital Smell?

If you stepped into the hospital, you can immediately notice a peculiar corrosive smell? What makes hospital to smell like that? Is it the smell of diseases? No, it is the smell of disinfectant that hospitals prefer to use to make the place clean and free from bacteria and viruses.

Louis Pasteur first observe that phenol kills bacteria and he used this substance to produce vaccines. This observation lead to develop the first antiseptic wound treatment in 1867.By spraying infected open leg fractures with aqueous phenol solution (carbolic acid), he could lower the mortality from such wounds from 60% to 10%. As a result carbolic acid was used over decades for the disinfection of hospitals and operating theaters, causing the typical unpleasant hospital smell.

Some of the other disinfectant used in hospitals which gives specific odor are given below.

Disinfectant used in Hospitals

 

The sense of smell is not considered adequate warning for the presence of hazardous materials. By the time you smell a chemical you are being exposed. In addition, many chemicals cause olfactory fatigue and deaden the sense of smell. Another substance which is responsible for hospital smell is iodoform. It is produced from ketones and is a pale yellow insoluble solid with a sweet antiseptic “hospital smell”.

Exposure to phenol through inhalation also leads to adverse health effects. Inhalation causes severe irritation of the respiratory tract with coughing, burns and breathing difficulty. Hospital smell a medley of sanitizers, chemicals and bodily fluids that causes many people to experience a vague sense of dread, unease and stress.

The Chemistry behind Artificial Sweeteners

Natural sweeteners for example, sucrose and fructose add to calorie intake and therefore may people prefer to use artificial sweeteners. Many people find artificial sweeteners to be an attractive alternate to the natural and calorie contributing counterparts. No acceptable general theory can explain what makes certain compounds taste sweet and precisely how they interact with human taste sensing systems.

Natural Sweeteners also increase the glucose level in persons suffering from diabetes. So to assist in weight loss and to manage diabetes mellitus people have been looking for artificial sweetener. These are duplicates of sugar and have less food energy called non-nutritive sweeteners. Some commonly used sugar substitutes are given below.

• Saccharin – Saccharin is about 550 times sweeter than sucrose. It is not biodegradable ad is excreted as such in urine. It has no calorie content. It has been proved to be a life saver for countless diabetes and is useful for those who need to control intake of calories.
• Aspartame – It is most widely used artificial sweetener. It is a methyl ester of a dipeptide unrelated to any carbohydrate. Aspartame is about 100 times as sweet as sucrose. It decomposes at baking or cooking temperature and hence can be used in cold foods and soft drinks.
• Cyclamate – It is low calorie additives with an intense sweetness. It was water soluble, cheaply produced and for almost two decades in combination with saccharin in sweet N low until evidence surfaced that it caused cancer in mice if consumed in vast amounts. It was banned by FDA in 1969 however, it is still available in some countries.
• Sucralose – It is relatively new artificial sweetener that was approved for use in USA. It is proposed to be as 600 times sweeter than sucrose. It is produced by chlorinating sucrose.

Does artificial sweeteners provide any benefits to humans? Is it safe for the environment? Have any long term studies been conducted on human consuming this artificial sugar? Unfortunately the answer to all of these questions is a resounding no! Better avoid using artificial sweeteners.

How To Succeed in Organic Chemistry Class?

Organic chemistry is often considered the hardest chemistry class. It's not that it's impossibly complicated, but there is a lot to absorb, in both the lab and classroom, plus you can expect to do some memorization to succeed at exam time. If you're taking o-chem, don't stress! Here are survival tips to help you learn the material and succeed in the class.

1) Choose How To Take Organic Chemistry

Are you more of a mental sprinter or is distance running your style? Most school offer organic chemistry one of two ways. You can take the year-long course, broken into Organic I and Organic II. This is a good choice if you need time to digest and learn material or master lab protocols. It's a good choice if you tend to ask a lot of questions, because your instructor will be able to take the time to answer them. Your other option is to take organic over the summer. You get the whole shebang in 6-7 weeks, sometimes with a break in the middle and sometimes straight through, start to finish.

If you're more of a cramming, run-to-the-finish type of student, this may be the way to go. You know your study style and level of self-discipline better than anyone else. Choose the learning method that works for you.

2) Make Organic Chemistry a Priority

Your social life may take a hit while you're taking organic. It won't be your first chemistry class, so you already expect that. Try to avoid taking other challenging courses at the same time. There are only so many hours in the day to work problems, write lab reports, and study. If you load your schedule with sciences, you're going to get pressed for time. Plan to give time to organic. Set aside time to read the material, do the homework, and study. You'll also need some downtime to relax. Getting away from it for a while really helps the material "click". Do not expect to just go to class and lab and call it a day. One of the biggest survival tips is to plan your time.

(3) Review Before and After Class

I know... I know... it's a pain to review general chemistry before taking organic and to review notes before the next class. Reading the textbook? Agony. Yet, these steps truly help because they reinforce material. Also, when you review the subject, you may identify questions to ask at the beginning of class. It's important to understand each part of organic because topics build on those you have already mastered. Continue reading..

3 Ways to Find the Volume in a Test Tube

Finding the volume of a test tube or NMR tube is a common chemistry calculation, both in the lab for practical reasons and in the classroom to learn how to convert units and report significant figures. Here are three ways to find the volume.

Calculate Density Using Volume of a Cylinder

A typical test tube has a rounded bottom, but NMR tubes and certain other test tubes have a flat bottom, so the volume contained in them is a cylinder. You can get a reasonably accurate measure of volume by measuring the internal diameter of the tube and the height of the liquid.

The best way to measure the diameter of a test tube is to measure the widest distance between the inside glass or plastic surfaces. If you measure all the way from edge to edge, you'll include the test tube itself in your measurements, which isn't correct.

Measure the volume of the sample from where it starts at the bottom of the tube to the base of the meniscus (for liquids) or the top layer of the sample. Don't measure the test tube from the bottom of the base to where it ends.

Use the formula for the volume of a cylinder to perform the calculation:

V = pi r^2 h

where V is volume, pi is 3.14 or 3.14159, r is the radius of the cylinder and h is the height of the sample

The diameter (which you measured) is twice the radius (or radius is one-half diameter), so the equation may be rewritten:

V = pi (1/2 d)^2 h

where d is diameter.

Example Volume Calculation

Let's say you measure an NMR tube and find the diameter to be 18.1 mm and height to be 3.24 cm. Calculate the volume. Report your answer to the nearest 0.1 ml.

First, you'll want to convert the units so they are the same. Please use cm as your units, because a cubic centimeter is a milliliter! This will save you trouble when it comes time to report your volume. Continue reading..

The Chemistry that brews in your Cup of Coffee

Does coffee have anything to do with chemistry? Yes it is. We often think chemistry is made of explosions and color changing liquids and those incredibly hard to pronounce chemical names. Well chemistry is all those things and much more. It is about the interactions of atoms and molecules. Chemistry is all around us all the time. Making coffee is chemistry.

Brewing coffee is half art and half chemistry. The exact portions of ground coffee to water, the water temperature and the water contact time with the grounds ass affect the flavor of the final coffee. Brewing of coffee depends on two factors, first the heat contained in the brewing water has a big influence on the extraction. Hot water with high energy can extract more coffee solids faster than colder water because energy facilitates molecular movement faster.

Coffee contains a complex combination of carbohydrates, lipids, proteins, amino acids, nucleic acids, vitamins, inorganic compounds, alkaloids and volatile compounds. In all coffee contains more than 1000 different chemical compounds around 800 are volatile chemicals that dissipate rapidly. Coffee rich aroma results from the mixture of about 50-60 volatile chemicals released during brewing.

Water acts as a solvent doing the work of extracting the flavors in the coffee during the brewing process. This is where the quality of the water plays important role as the hardness and the mineral content can significantly affect how coffee brews. Harder water seems to change the rate at which the soluble in the coffee go into solution. Hard water does a poor job of brewing coffee.

What is a Molecule?

 

What Is a Molecule?

The terms molecule, compound, and atom can be confusing! Here's an explanation of what a molecule is (and is not) with some examples of common molecules.

Molecules form when two or more atoms form chemical bonds with each other. It doesn't matter if the atoms are the same or are different from each other.

Examples of Molecules

Molecules may be simple or complex. Here are examples of common molecules:

• H2O (water)
• N2 (nitrogen)
• O3 (ozone)
• CaO (calcium oxide)
• C6H12O6 (glucose, a type of sugar)

Molecules Versus Compounds

Molecules made up of two or more elements are called compounds. Water, calcium oxide, and glucose are molecules that are compounds. All compounds are molecules; not all molecules are compounds.

What Is Not a Molecule?

Single atoms of elements are not molecules. A single oxygen, O, is not a molecule. Continue reading..

What makes a Neon Light Glow?

Have you ever seen a colorful glowing tube of light in a store’s front window or outside a restaurant, the glow is due to some noble gases which emits. These glowing tubes are called neon lights. Neon lights are commonly seen on advertising hoardings, outside shops, restaurants and cinemas. Their color is so distinctive that to most people the word “neon” has come to mean a dark pink glow, although neon is merely an elemental rare gas element.

The technology behind how a neon light works is very different from that of normal incandescent lights. Electroluminescence or the conversion of electricity directly into light is the operating principle of neon lights.

• A neon light consists of a glass tube filled with a noble gas at a very low pressure.
• Neon lights containing the other noble gases are also common.
• Neon is wholly colorless if placed in a normal flask, yet to glow of an eon light is visible even at night, so its color cannot be due to the way it interacts with light.
• Metal plates called electrodes are fitted to each end of the gas filled tube.
• When the neon light is plugged into an electricity supply, an electrical current flows through the tube between the electrodes.
• Gas discharge takes place and collisions involving mobile electrons generally elastic. They bounce like a ball off a wall.
• An extreme voltage ionizes the neon to form a plasma of Ne⁺ and e^-. Inelastic collision between Ne+ and neutral atom allow for the release of energy as visible light.
• Neon atoms are neutral, but ionization of the gas phase neon forms Ne+ ions, so the tube contains a mixture of electrons and ions.

Ne⁰(g) --> Ne⁺(g) + e^-

Thus neon light glows because they emit light under suitable conditions. All the light we see id a result of electrons jumping between electron shells. Subsequent relaxation of the electron releases a photon of light. It is this emission that we see as the bright glow of neon light.