Acids and Bases Lessn 3 The ph f a slutin is defined as the negative lgarithm, t the base ten, f the hydrnium in cncentratin. In a neutral slutin at 25 C, the hydrnium in and the hydrxide in cncentratins are bth 1.0 10-7 ml/l. Thus, the ph f a neutral slutin is 7. ph = lg10(1.0 10-7 ) = 7.00 Example: What will be the ph f an aqueus slutin cntaining 0.040 M sdium hydrxide? Example: What is the hydrnium in cncentratin f a slutin with a ph f 2.50? Determining poh poh is anther way t express hw acidic r basic a slutin is.
It s basically just the ppsite f ph. ph + poh = 14.00 poh = lg10[oh ] poh = 14.00 ph Example: A slutin has a ph f 3.00, is it an acid r base? What wuld be the poh f the same slutin? Example: An aqueus slutin cntaining 0.040 M sdium hydrxide has the fllwing poh: Neutralizatin See ph Assignment Recall: neutralizatin refers t the duble displacement reactin which ccurs when an acid is mixed with a base, the prducts f neutralizatin are a (inic cmpund) and. this is nly true fr a STRONG acid and a STRONG base. Ex. HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) Nw if we lk at the net inic equatin: H + (aq) + OH - (aq) H2O(l) This is because when we have equal mles f acid and base, we will have a slutin (a ph f 7) If yu did nt have a BOTH strng acids and bases, the ph when the mles are equal wuld nt be exactly 7. It wuld depend n the Ka and Kb f the acids and bases Strng acid + Strng base ph = 7 (neutral) when neutralized. Strng acid + weak base ph < 7 (acidic) when neutralized. Weak acid + strng base ph > 7 (basic) when neutralized.
Acid-Base Titratins T achieve neutralizatin we use a prcess called titratin. One reactant is carefully added t anther reactant until the tw have cmbined in their exact stichimetric prprtins. When the mles are equal this is called the equivalency pint The bjective f a titratin is usually t find the number f mles r grams, the cncentratin, r the percentage f the analyte (the substance we are lking fr) in a sample. This is usually dne by measuring the precise vlume and cncentratin f a titrant (the slutin being added frm the burette) needed t react cmpletely with the analyte. Stichimetric relatinships are then used t determine the quantity f analyte that was present frm the number f mles (vlume mlarity) f titrant added. During titratin, the slutin reaches an pint where the mles f acid equals the mles f the base (this des nt necessarily mean ph = 7). During a titratin, the equivalency pint is shwn using an When ding titratins we use a because it is really accurate at measuring vlumes. When perfrming a titratin yu add yur titrant t the analyte (with indicatr) until there is ONE DROP that has a clur change. Srting ut sme cnfusing terms Frm: http://www.chemguide.c.uk/physical/acidbaseeqia/phcurves.html The term "neutral pint" is best avided. The term "equivalence pint" means that the slutins have been mixed in exactly the right prprtins accrding t the equatin. The term "end pint" is where the indicatr changes clur. As yu will see n the page abut indicatrs, that isn't necessarily exactly the same as the equivalence pint. Simple ph curves Althugh yu nrmally run the acid frm a burette int the alkali in a flask, yu may need t knw abut the titratin curve fr adding it the ther way arund as well. Alternative versins f the curves have been described in mst cases. Titratin curves fr strng acid v strng base (alkali) We'll take hydrchlric acid and sdium hydrxide as typical f a strng acid and a strng base.
Running acid int the alkali Running alkali int the acid Titratin curves fr strng acid v weak base This time we are ging t use hydrchlric acid as the strng acid and ammnia slutin as the weak base. Running acid int the alkali Running alkali int the acid Titratin curves fr weak acid v strng base We'll take ethanic acid and sdium hydrxide as typical f a weak acid and a strng base. Running acid int the alkali Running alkali int the acid
Titratin curves fr weak acid v weak base The cmmn example f this wuld be ethanic acid and ammnia. CH3COOH + NH3 CH3COONH4 It s happens that these tw are bth abut equally weak - in that case, the equivalence pint is apprximately ph 7. Running acid int the alkali: A summary f the imprtant curves The way yu nrmally carry ut a titratin invlves adding the acid t the alkali. Here are reduced versins f the graphs described abve s that yu can see them all tgether.
Acid-Base Indicatrs Indicatrs are chemicals that appear different in acids and bases. Indicatrs are weak acids r bases Indicatrs change clurs when the ph (r poh) f a slutin reaches a certain, s they can be used t determine the ph f slutins. Indicatrs change clur based n their wn equilibrium In - (aq) + H + (aq) HIn(aq) Indicatrs dn't change clur sharply at ne particular ph (given by their pkind). Instead, they change ver a narrw range f ph. Assume the equilibrium is firmly t ne side, but nw yu add smething t start t shift it. As the equilibrium shifts, yu will start t get mre and mre f the secnd clur frmed, and at sme pint the eye will start t detect it.
Example - A given slutin turns methyl range yellw, litmus blue, and phenlphthalein pink. What is the apprximate ph f the slutin? Methyl range in yellw when ph is abve Litmus is blue when ph is abve, and Phenlphthalein is pink when ph is abve. Indicatr methyl range litmus ph range 3.2-4.4 5.8-8.0 Clur change red t yellw red t blue Therefre the slutin wuld have t have a ph phenlphthalein 8.2-10.0 clurless t pink Determining Equivalence Pint Hw des ne knw when the equivalence pint has been reached when titrating? Measure the ph electrnically using ph electrdes and cmputerized titratrs. If a titratin is fllwed using a ph meter the results can be graphed and a titratin curve results: This curve represents the additin f a base t an acidic slutin. The ph changes very slwly until the equivalence pint is apprached. As the equivalence pint is apprached, the ph changes very rapidly with the additin f just ne r tw drps f additinal base. The equivalence pint is the center f the straight/vertical prtin f the curve. Indicatrs are the methd f chice in student labs because they are cheap and easy t use. They are als used in the field, where instrumentatin is limited. Lking at the equivalence pint fr this graph, determine what a gd indicatr might be: Plyprtic acids If the titratin is dne with a plyprtic acid, like phsphric acid, the fllwing titratin curve results: The curve has three stages representing the equivalence pint fr each f the three hydrgen ins in H3PO4. Determine the apprximate ph at each equivalence pint in the titratin with H3PO4.
Indicatrs & Le Chatelier s Principle: Litmus is a weak acid. It has a seriusly cmplicated mlecule which we will simplify t HLit. The "H" is the prtn which can be given away t smething else. The "Lit" is the rest f the weak acid mlecule. There will be an equilibrium established when this acid disslves in water. Taking the simplified versin f this equilibrium: The un-inised litmus is red, whereas the in is blue. Nw use Le Chatelier's Principle t wrk ut what wuld happen if yu added hydrxide ins r sme mre hydrgen ins t this equilibrium. Adding hydrxide ins: Adding hydrgen ins: ph Ranges f Cmmn Indicatrs
(Lide, David R., ed. CRC Handbk f Chemistry and Physics: A Ready-Reference f Chemical and Physical Data. 87th ed. Bca Ratn: Taylr & Francis Grup, 2006.) Chsing The Right Indicatr When chsing an indicatr t use fr a titratin yu need t cnsider where abut the equivalence pint will lie. This is where yur knwledge f ph curves fr strng/weak acids/bases cmes in. T determine the crrect indicatr: 1. Determine which type (strng r weak) f acid and base yu are wrking with. 2. Draw a rugh sketch f the ph curve fr that titratin. 3. Identify the apprximate ph f the equivalence pint. 4. Lk at yur table f cmmn indicatrs and chse ne that changes clur ver that ph range. Remember that the equivalence pint f a titratin is where yu have mixed the tw substances in exactly equatin prprtins. Yu bviusly need t chse an indicatr which changes clur as clse as pssible t that equivalence pint. That varies frm titratin t titratin. Examples: Which indicatr wuld yu chse fr the titratin in the fllwing diagrams?
Strng acid v strng base Strng acid v weak base Weak acid v strng base Weak acid v weak base
ph Name: 1. Calculate the ph f each f the fllwing: a) an aqueus slutin that has a hydrnium in cncentratin f 3.0 10 3 M b) an aqueus slutin that has a hydrxide in cncentratin f 6.0 10 4 M c) an aqueus slutin cntaining 0.0020 M barium hydrxide d) 250.0 ml f an aqueus slutin cntaining 1.26 g f nitric acid 2. Calculate the hydrnium cncentratin f a) 100.0 ml f an aqueus slutin cntaining 0.60 g f sdium hydrxide b) a bld sample with a ph f 7.40 c) range juice with a ph f 3.20 3. Calculate the ph f a slutin created by disslving ne pellet (0.10 g) f sdium hydrxide in 1.0 L f pure water. 4. The ph f an aqueus slutin f NaOH is 12.9. What is the mlarity f the slutin?
5. What is the ph f a 0.00125 M HBr slutin? If 175 ml f this slutin is diluted t a ttal vlume f 3.00 L, what is the ph f the diluted slutin? 6. A cntainer is labeled 500.0 ml f 0.00157 M nitric acid slutin. A chemist finds that the cntainer was nt sealed and that sme evapratin has taken place. The vlume f slutin is nw 447.0 ml. a. What was the riginal ph f the slutin? b. What is the ph f the slutin nw? 7. An acetic acid slutin has a ph f 4.0. What are the [H3O + ] and [OH ] in this slutin? 8. What is the ph f a 0.000460 M slutin f Ca(OH)2? 9. A slutin f HCl has a ph f 1.50. Determine the ph f the slutins made in each f the fllwing ways. a. 1.00 ml f the slutin is diluted t 1000.0 ml with water. b. 25.00 ml is diluted t 200.0 ml with distilled water. c. 18.83 ml f the slutin is diluted t 4.000 L with distilled water.
d. 1.50 L is diluted t 20.0 kl with distilled water. 10. A hydrchlric acid slutin has a ph f 1.70. What is the [H3O + ] in this slutin? Cnsidering that HCl is a strng acid, what is the HCl cncentratin f the slutin? 11. Calculate the ph f the fllwing aqueus slutins: a. an aqueus slutin cntaining 3.0 10 5 M hydrnium ins b. an aqueus slutin cntaining 6.5 10 4 M hydrxide ins c. the slutin btained when 7.2 g hydrgen chlride is disslved in 5.0 L f water d. a 500.0 ml aqueus slutin cntaining 1.0 g sdium hydrxide 12. What are the hydrnium in and the hydrxide in cncentratins f the fllwing: a) a slutin with a ph = 5.00 b) a slutin with a ph = 9.55 13. Fill in the blanks in the fllwing table. Fr each slutin indicate whether the slutin is acidic r basic. H 4.0 p poh [H3O + ] [OH ] A r B 11.6 1.8 10 9
3.5 10 2 14. Calculate the ph f the slutin btained when 50.00 ml f 0.150 M Hydrchlric acid is added t 75.0 ml f a 0.111 M aqueus sdium hydrxide slutin Indicatrs Assignment Name: 1 2 3 a. b. 4
Name: Indicatr Assignment 2 1. Multiple Chice: In the titratin f a weak acid with a strng base, which indicatr wuld be the best chice? a. Methyl range b. Brmscresl Green c. Phenlphthalein 2. Which indicatr (identified by a letter) culd be used t titrate aqueus NH3 with HCl Slutin? Indicatr Acid Range Clr Clur changes ph Basic Range Clur a. Pink 1.2 2.8 Yellw b. Blue 3.4 4.6 Yellw c. Yellw 6.5 7.8 Purple d. Clurless 8.3 9.9 Red e. Nne f these 3. Cnsider the titratin f the pairs f aqueus acids and bases listed n the left. Fr which pair is the ph at the equivalence pint stated incrrectly? Acid Base Pair ph at Equivalence Pint HCl + NH3 Less than 7 HNO3 + Ca(OH)2 Equal t 7 HClO4 + NaOH Equal t 7 HClO + NaOH Less than 7 CH3COOH + KOH Greater than 7 4. The fllwing titratin curve is the kind f curve expected fr the titratin f a acid with a base. a. strng, strng b. weak, strng c. weak, weak d. strng, weak 5. Which clur des the universal indicatr turn in hydrchlric acid? Universal indicatr: ph 1 4 red, ph 5 6 yellw, ph 7 green, ph 8 9 blue, ph 10 14 purple. a. Red b. Yellw c. Green d. Blue e. Purple 6. Which f the fllwing pieces f apparatus wuld yu NOT use in a titratin? a. A gas jar c. Erlenmeyer flask b. Burette d. Stand with clamp 7. fr the fllwing curves, determine the best indicatr t use: a. brmthyml blue b. phenlphthalein
c. brmcresl green d. methyl range