how to know what is a strong acid

STRONG AND WEAK ACIDS This page explains the terms strong and weak as applied to acids. As a part of this it defines and explains what is meant by pH, One thousanda and pKa. It is important that you don't confuse the words strong and weak with the terms concentrated and dilute. As you volition encounter below, the forcefulness of an acid is related to the proportion of it which has reacted with water to produce ions. The concentration tells you nearly how much of the original acid is dissolved in the solution. It is perfectly possible to accept a concentrated solution of a weak acrid, or a dilute solution of a strong acid. Read on . . . Strong acids Explaining the term "strong acid" Nosotros are going to use the Bronsted-Lowry definition of an acid.
	Annotation:If you don't know what the Bronsted-Lowry theory of acids is, you should read near theories of acids and bases on another page in this section. You lot don't need to spend time reading about Lewis acids and bases for the purposes of this present page. Use the Dorsum button on your browser when you are set up to return to this page.
When an acid dissolves in h2o, a proton (hydrogen ion) is transferred to a water molecule to produce a hydroxonium ion and a negative ion depending on what acid yous are starting from. In the general example . . . These reactions are all reversible, just in some cases, the acrid is and then proficient at giving away hydrogen ions that nosotros tin think of the reaction as beingness one-way. The acid is nearly 100% ionised. For example, when hydrogen chloride dissolves in water to make hydrochloric acid, so little of the reverse reaction happens that we can write: At any one time, well-nigh 100% of the hydrogen chloride will have reacted to produce hydroxonium ions and chloride ions. Hydrogen chloride is described every bit a strong acid. A strong acid is one which is near 100% ionised in solution. Other common strong acids include sulphuric acid and nitric acid. You may find the equation for the ionisation written in a simplified form: This shows the hydrogen chloride dissolved in the water splitting to give hydrogen ions in solution and chloride ions in solution. This version is often used in this work just to make things look easier. If you lot use it, remember that the water is really involved, and that when yous write H+ (aq) what you really mean is a hydroxonium ion, HthreeO+.
	Note:You lot should observe out what your examiners adopt on this. You are unlikely to discover this from your syllabus, merely should look at recent exam papers and mark schemes. If you are doing a Great britain-based exam and oasis't got copies of your syllabus and past papers, you should have! Follow this link to detect out how to go concord of them.
Potent acids and pH pH is a measure out of the concentration of hydrogen ions in a solution. Strong acids like hydrochloric acid at the sort of concentrations you unremarkably employ in the lab have a pH around 0 to one. The lower the pH, the college the concentration of hydrogen ions in the solution. Defining pH
	Note:If you lot are asked to define pH in an test, simply write down the expression in black. Never try to ascertain it in words - it is a waste of time, and you are too likely to miss something out (like mentioning that the concentration has to exist in mol dm-3). In the expression, higher up, the foursquare brackets imply that, so you lot don't need to mention it.
Working out the pH of a potent acid Suppose you had to piece of work out the pH of 0.one mol dm-iii hydrochloric acid. All yous have to do is work out the concentration of the hydrogen ions in the solution, then utilize your estimator to convert information technology to a pH. With strong acids this is easy. Muriatic acid is a stiff acid - nigh 100% ionised. Each mole of HCl reacts with the water to give 1 mole of hydrogen ions and 1 mole of chloride ions That means that if the concentration of the acid is 0.1 mol dm-3, so the concentration of hydrogen ions is also 0.ane mol dm-three. Use your computer to catechumen this into pH. My calculator wants me to enter 0.one, and then printing the "log" push button. Yours might desire you to do it in a different order. You demand to find out! logx [0.one] = -ane But pH = - logten [0.ane] - (-1) = ane The pH of this acid is 1.
	Notation:If you desire more examples to look at and to try yourself (with fully worked solutions given), y'all may be interested in my chemistry calculations volume. This also includes the slightly more than confusing problem of converting pH back into hydrogen ion concentration.
Weak acids Explaining the term "weak acid" A weak acid is ane which doesn't ionise fully when it is dissolved in h2o. Ethanoic acid is a typical weak acid. It reacts with water to produce hydroxonium ions and ethanoate ions, but the back reaction is more than successful than the frontward ane. The ions react very easily to reform the acid and the h2o. At any i fourth dimension, simply about 1% of the ethanoic acid molecules have converted into ions. The rest remain every bit simple ethanoic acid molecules. Most organic acids are weak. Hydrogen fluoride (dissolving in water to produce hydrofluoric acrid) is a weak inorganic acrid that yous may come across elsewhere.
	Note:If you are interested in exploring organic acids further, you will find them explained elsewhere on the site. It might exist a good idea to read the rest of this page first, though. If you want to know why hydrogen fluoride is a weak acrid, you tin find out by following this link. These pages are in completely different parts of this site. If yous follow either link, use the BACK button to render to this current folio.
Comparing the strengths of weak acids The position of equilibrium of the reaction between the acrid and water varies from one weak acid to another. The farther to the left it lies, the weaker the acid is.
	Note:If you don't understand about position of equilibrium follow this link before you lot go any further. You are also going to need to know about equilibrium constants, Kc for homogeneous equilibria. There is no point in reading any more of this page unless you do! If you follow either link, use the BACK button to return to this electric current page.
The acrid dissociation constant, Ka You can get a mensurate of the position of an equilibrium by writing an equilibrium abiding for the reaction. The lower the value for the constant, the more than the equilibrium lies to the left. The dissociation (ionisation) of an acrid is an instance of a homogeneous reaction. Everything is present in the same phase - in this case, in solution in water. You can therefore write a unproblematic expression for the equilibrium constant, Chiliadc. Here is the equilibrium again: Y'all might wait the equilibrium constant to be written every bit: However, if you recall about this carefully, in that location is something odd well-nigh it. At the bottom of the expression, you have a term for the concentration of the water in the solution. That'south not a trouble - except that the number is going to be very big compared with all the other numbers. In ane dm3 of solution, there are going to be about 55 moles of water.
	Note:1 mole of water weighs 18 1000. 1 dm3 of solution contains approximately 1000 k of water. Divide k by eighteen to get approximately 55.
If you lot had a weak acrid with a concentration of about one mol dm-3, and but near 1% of it reacted with the h2o, the number of moles of h2o is but going to fall by about 0.01. In other words, if the acid is weak the concentration of the water is virtually abiding. In that case, there isn't a lot of indicate in including it in the expression equally if it were a variable. Instead, a new equilibrium constant is defined which leaves information technology out. This new equilibrium abiding is called Ka. You may observe the Ka expression written differently if you lot piece of work from the simplified version of the equilibrium reaction: This may be written with or without state symbols. It is actually exactly the same as the previous expression for Ka! Remember that although we often write H+ for hydrogen ions in solution, what nosotros are actually talking most are hydroxonium ions. This 2d version of the Ka expression isn't every bit precise as the first one, but your examiners may well have it. Find out! To have a specific mutual instance, the equilibrium for the dissociation of ethanoic acid is properly written as: The Thoua expression is: If yous are using the simpler version of the equilibrium . . . . . . the Chiliada expression is:
	Note:Because you are likely to come across both of these versions depending on where you read well-nigh Ma, you would be wise to become used to using either. For examination purposes, though, employ whichever your examiners seem to adopt.
The table shows some values of Thoua for some simple acids: acrid Ka (mol dm-3) hydrofluoric acid 5.vi x 10-4 methanoic acid 1.vi 10 x-iv ethanoic acrid one.7 x 10-v hydrogen sulphide viii.nine x 10-8 These are all weak acids because the values for Ka are very minor. They are listed in social club of decreasing acid strength - the Ma values get smaller equally you go downwardly the table. However, if you aren't very happy with numbers, that isn't immediately obvious. Because the numbers are in two parts, in that location is too much to retrieve about quickly! To avoid this, the numbers are oft converted into a new, easier form, called pKa. An introduction to pKa pKa bears exactly the same relationship to Thoua as pH does to the hydrogen ion concentration: If you utilise your calculator on all the One thousanda values in the table above and convert them into pKa values, you lot get: acid Chiliada (mol dm-three) pKa hydrofluoric acid v.six x 10-4 iii.3 methanoic acid 1.6 x 10-4 3.eight ethanoic acrid 1.seven x 10-v four.eight hydrogen sulphide 8.9 x 10-8 seven.1
	Note:Find that unlike Ka, pKa doesn't take any units.
Discover that the weaker the acid, the larger the value of pKa. Information technology is now easy to see the trend towards weaker acids as you go down the table. Remember this: The lower the value for pKa, the stronger the acrid. The higher the value for pKa, the weaker the acid.
	Notation:If y'all need to know about Ka and pKa, you are quite likely to demand to be able to practice calculations with them. You will probably demand to exist able to summate the pH of a weak acid from its concentration and Thoua or pKa. You may demand to reverse this and calculate a value for pKa from pH and concentration. I tin can't help you with these calculations on this site, only they are all covered in detail in my chemistry calculations book.
Questions to test your agreement If this is the offset set of questions you lot accept done, please read the introductory folio before yous showtime. You will need to employ the Dorsum Button on your browser to come back hither subsequently. questions on strong and weak acids answers Where would you like to go at present? To the acid-base equilibria bill of fare . . . To the Concrete Chemical science bill of fare . . . To Main Carte du jour . . . © Jim Clark 2002 (modified November 2013)

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