- WELCOME TO ACIDS AND BASES PART 6
IN THIS PART, WE'RE GOING TO LOOK AT PH CALCULATIONS.
ALL RIGHT. SO THIS WILL BUILD OFF OUR UNDERSTANDING OF KW.
SO REMEMBER THAT WHEN WE HAVE A HIGH HYDRONIUM CONCENTRATION,
WE DESCRIBE THE SOLUTION AS ACIDIC
AND WHEN WE HAVE A LOW HYDRONIUM ION CONCENTRATION,
I GUESS YOU PUT OUT THERE, THEN THAT SOLUTION WOULD BE BASIC.
AND WHEN THE HYDRONIUM CONCENTRATION IS IN BALANCE
WITH THE HYDROXIDE, IT'S NEUTRAL.
BUT AS WE LOOK AT THESE CONCENTRATIONS,
THEY'RE VERY CLUNKY.
WE HAVE TO USE SCIENTIFIC NOTATION
AND THEY'RE A BIT MESSY OR AWKWARD.
SO WHAT WE LIKE TO DO IN CHEMISTRY
IS WE TAKE THE HYDROGEN ION CONCENTRATION
AND WE'RE GOING TO TAKE THE NEGATIVE LOG.
SO THE PH MEANS, WHERE YOU SEE THE WORD P,
THAT MEANS WE TAKE THE NEGATIVE LOG
AND THEN THE H PART REPRESENTS THE HYDRONIUM ION CONCENTRATION.
AND CHEMISTS LIKE TO TAKE THE NEGATIVE LOG
OF ALL KINDS OF THINGS.
SO ANY TIME YOU SEE THE P IN FRONT OF A SYMBOL,
YOU'LL KNOW THAT SOMEBODY HAS TAKEN THE NEGATIVE LOG.
NOW, ALGEBRAICALLY, WE CAN REWRITE THIS
AND USING OUR UNDERSTANDING OF LOGARITHMS
AND WE COULD CALCULATE THE HYDRONIUM ION CONCENTRATION
AS 10 TO THE -PH, OR THE ANTILOG.
SO IF YOU NEED HELP WITH LOGARITHMS,
SEE A TUTOR OR YOUR PROFESSOR, IF IT'S ME, DURING OFFICE HOURS,
SO THESE ARE OUR TWO MAIN FUNDAMENTAL CALCULATIONS.
IF WE KNOW THE HYDRONIUM ION CONCENTRATION,
WE CAN TAKE THE NEGATIVE LOG AND CALCULATE THE PH OR VICE VERSA.
IF WE'VE MEASURED THE PH WITH AN ELECTRODE OR SOMETHING,
WE CAN DETERMINE THE HYDRONIUM ION CONCENTRATION.
SO THESE ARE TWO CALCULATIONS AND FORMULAS
THAT YOU WANT TO MEMORIZE.
SO THE IMPORTANT THING TO THINK ABOUT HERE THOUGH
IS IT IS A LOG SCALE.
SO WHEN WE HAVE A LOW PH, THAT'S GOING TO BE ACIDIC
AND WHEN WE HAVE A HIGH PH, THAT'S BASIC. ALL RIGHT?
AND THEN THIS WOULD BE A LOW PH. ALL RIGHT.
AND THEN OUT OF GENERAL INTEREST...
SO ANYTHING AROUND--REMEMBER ANYTHING AROUND PH SEVEN,
WE WOULD SAY IS IN THE NEUTRAL RANGE.
SO ANYTHING WITH A PH BELOW SEVEN IS CONSIDERED ACIDIC
AND ANYTHING WITH A PH ABOVE SEVEN, WE WOULD CONSIDER BASIC.
AND THEN OF COURSE PH AT SEVEN IS NEUTRAL.
SO IF YOU'RE INTERESTED IN ANY OF THESE COMMON MATERIALS,
THERE'S SOME INFORMATION FOR YOU.
WE'RE JUST GOING FOR THE MAIN TREND THERE.
SO NOW, LET'S GO TO THE NEXT PAGE AND PRACTICE
USING THESE FORMULAS.
SO HERE ARE THE TWO EQUATIONS THAT YOU WANT TO MEMORIZE,
THE PH IS THE NEGATIVE LOG OF THE HYDROGEN ION
OR REMEMBER THE HYDRONIUM ION CONCENTRATION.
SO REMEMBER THAT YOU'RE NOT ALWAYS GOING TO BE
READING INFORMATION FROM PEOPLE THAT ARE CAREFUL.
SO IN WATER, WE SHOULD BE WRITING HYDRONIUM
BUT THERE ARE NO HYDRONIUM COPS OUT THERE TO ENFORCE IT,
SO THE POINT HERE IS WE USE THIS FORMULA, THAT WE'RE GIVEN,
H3O+, RIGHT.
WE CAN CALCULATE PH AND THEN VICE VERSA,
AS WE SHOWN ON THE PREVIOUS PAGE, WE COULD REARRANGE THIS
AND THEN THE POWER HERE IS,
GIVEN THE PH, WE CAN CALCULATE THESE CONCENTRATIONS.
OKAY. SO LET'S PRACTICE THAT.
PANCREATIC FLUID HAS A HYDRONIUM CONCENTRATION
OF 10 TO THE -8 MOLAR.
WHAT IS THE PH?
SO THE PH IS THE -LOG OF 10 TO THE -8 MOLAR.
SO YOU WOULD PLUG INTO YOUR CALCULATOR,
10 TO THE -8, TAKE THE LOG AND REVERSE THE SIGN
AND YOU SHOULD BE ABLE TO DO THIS REALLY IN YOUR HEAD
AND SO WE WOULD SAY THAT THE PH IS 8
AND WE RECOGNIZE THAT 8 IS GREATER THAN 7.
SO THE PANCREATIC FLUID IS BASIC, SLIGHTLY BASIC.
OUR BLOOD IDEALLY HAS A PH OF 7.4.
SO NOW IN THIS EXAMPLE, WE WERE GIVEN THE PH
AND ASKED TO CALCULATE THE HYDRONIUM ION CONCENTRATION.
SO H3O+ IS GOING TO BE 10 TO THE -PH
WHICH WILL BE 10 TO THE -7.4.
SO HERE YOU CAN USE YOUR CARET SYMBOL ON YOUR CALCULATOR
AND THE CALCULATOR WILL TELL YOU THAT WE WOULD GET
4 X 10 TO THE -8 MOLAR H+.
OKAY. SO SIG FIGS.
I'LL TALK A LITTLE BIT ABOUT SIG FIGS ON THE NEXT PAGE
BUT FOR RIGHT NOW, LET'S JUST FOCUS ON
WHEN TO USE THE VARIOUS EQUATIONS.
SO NOW LET'S BRING THIS ALL TOGETHER.
ORIGINALLY, WE LEARNED ABOUT KW AND NOW WE'VE LEARNED ABOUT PH
AND WE CAN TAKE THE -LOG OF ANYTHING.
SO THE POH WOULD SIMPLY BE THE -LOG OF THE HYDROXIDE
CONCENTRATION.
SO NOT A HUGE EMPHASIS FOR OUR COURSE
BUT JUST TO FLESH OUT YOUR KNOWLEDGE.
SO NOW WHAT WE COULD DO
IS WE COULD TAKE THE -LOG OF THE ENTIRE KW EXPRESSION
AND WHEN WE DO THAT, THE RESULTING EQUATION
WOULD BE THAT THE PH + THE POH = 14.
AND SO WE HAVE, SO THESE WOULD BE OUR THREE,
WHERE I'VE STARRED, THESE WOULD BE OUR THREE MAIN EQUATIONS
THAT WILL BE HELPFUL TO KNOW AND USE.
AND THEN THE FOURTH ONE
OF COURSE THEN WOULD BE THAT THE HYDROGEN ION CONCENTRATION
EQUALS 10 TO THE -PH.
SO FOR ACID-BASE CHEMISTRY,
THESE WOULD BE THE FOUR EQUATIONS THAT YOU WOULD FIND
HELPFUL AND THIS ONE, YOU SHOULD JUST BY ANALOGY.
OKAY. LET'S PUT THESE EQUATIONS TO WORK NOW
ON THE NEXT PAGE.
ALL RIGHT. SO THE FORMULAS ARE ALL RESUMMARIZED RIGHT HERE.
THESE ARE THE ONES THAT YOU REALLY WANT
TO BE COMFORTABLE WITH.
LET'S TALK BRIEFLY ABOUT SIG FIGS,
NOT A HUGE EMPHASIS FOR OUR CLASS BUT FOR THOSE OF YOU
WITH A CURIOUS MIND.
IF THE PH HAS TWO DIGITS TO THE RIGHT,
SO RIGHT HERE. RIGHT? TWO TO THE RIGHT,
THEN WHEN WE REPORT THE HYDROGEN ION CONCENTRATION,
WE WOULD HAVE ONE DIGIT TO THE RIGHT
AND IF YOU'RE REALLY CURIOUS ABOUT THIS, PLEASE COME SEE ME
ON OFFICE HOURS
BUT IT'S IT KIND OF MAKES PEOPLE'S HEAD HURT
AND WE HAVE MORE IMPORTANT THINGS TO LEARN. ALL RIGHT?
SO THEN BY ANALOGY, IF OUR PH HAS LESS SIG FIGS,
LET'S SAY WE ONLY HAVE ONE TO THE RIGHT,
SO THEN WHAT WOULD WE REPORT?
WE WOULD SIMPLY REPORT ONE DIGIT--ZERO TO THE RIGHT.
OKAY. SO THERE'S A LITTLE BRIEF EXPLANATION OF SIG FIGS.
IF YOU WANT TO KNOW MORE, PLEASE SEE ME ON OFFICE HOURS.
SO NOW, WHAT YOU LIKE TO DO IS YOU WANT TO TAKE SOME TIME
TO COMPLETE THIS TABLE.
SO IF YOU HAVE A GOOD TIME TO STOP THE VIDEO
AND PERFORM THE CALCULATIONS AND THEN COMPARE ANSWERS WITH ME
AND IN THE MEANTIME I'LL WORK THEM OUT FOR YOU.
SO H3O+, HYDRONIUM AND HYDROXIDE, RIGHT?
SO THIS IS GOING TO USE KW.
SO IF WE USE THE KW, WE WILL SEE THAT THE HYDRONIUM CONCENTRATION
IS GOING TO BE 10 TO THE -4 MOLAR.
ONCE WE HAVE THE HYDRONIUM CONCENTRATION,
WE CAN DETERMINE THE PH. THE PH IS THE -LOG
OF THE HYDRONIUM CONCENTRATION
AND SO BASED OFF HAVING TWO DIGITS HERE,
WE WOULD REPORT AS 4.00 AND THE PH IS LESS THAN SEVEN
SO THIS WOULD BE AN ACIDIC ENVIRONMENT.
OKAY. FOR THIS NEXT EXAMPLE, WE HAVE THE PH.
SO THIS TIME,WE WILL USE THAT THE HYDRONIUM ION CONCENTRATION
IS 10 TO THE -PH.
THAT WILL--I GUESS, I SHOULDN'T--WAIT....
SO THIS INFORMATION GIVES US THAT.
SO THE PH, WE CAN CALCULATE HERE AND SO WE WILL GET
1.0 X 10 TO THE -9 MOLAR FOR THE HYDRONIUM CONCENTRATION
AND ONCE WE HAVE THAT, THEN WE CAN USE THE KW.
SO MY ARROWS WERE A LITTLE FUNKY HERE, RIGHT?
SO FROM THE PH, WE GET THE HYDRONIUM.
FROM THE HYDRONIUM, WE CAN GET THE HYDROXIDE.
AND THEN THIS ONE, WE ALREADY KNOW,
THE PH IS GREATER THAN SEVEN, BASIC.
OKAY. SO WHEN YOU GET TO YOUR BIO CLASSES,
THEY DON'T LET YOU USE CALCULATORS
BUT THEY TEND TO USE COEFFICIENTS OF 1.
THERE'S ONE PROFESSOR I KNOW THAT WILL SOMETIMES USE
COEFFICIENT OF 5.
SO LET'S DO AN EXAMPLE OF THAT TO HELP YOU.
ALL RIGHTY. SO HERE WE HAVE THE HYDRONIUM CONCENTRATION.
SO FROM HERE WE CAN GO DIRECTLY TO THE HYDROXIDE CONCENTRATION
AND TO THE PH. RIGHT?
SO TO GET THE HYDROXIDE CONCENTRATION,
WE'RE GOING TO USE KW, SO 1 X 10 TO THE -14
DIVIDED BY 5.0 X 10 TO THE -3.
SO WHEN WE HAVE THE 1 DIVIDED BY 5,
THEN WE COULD THINK OF THIS AS 10 X -13 DIVIDED BY 5 X 10
TO THE -3.
YEAH, I DON'T LIKE THAT THOUGH.
WE'RE JUST GOING TO GO USE OUR CALCULATOR.
I SHOULDN'T MAKE THINGS UP WHEN I'M ON CAMERA.
ALL RIGHT. SO YOU DO THIS CALCULATING AND THE RESULT
THAT YOU'RE GOING TO GET FOR THE HYDROXIDE CONCENTRATION
IS 2.0 X 10 TO THE -12.
AND NOW WE TAKE THIS VALUE AND WE CAN PLUG IT STRAIGHT IN
FOR THE PH, RIGHT?
WE TAKE THE PH IS THE -LOG OF 5 X 10 TO THE -3 MOLAR
AND SO WE WOULD GET OUR PH TO BE 2.30
AND THAT'S LESS THAN SEVEN AND SO THAT'S ACIDIC.
OKAY. SO THERE YOU GO.
WE HAVE PRACTICED WORKING WITH ALL OF THE MOST IMPORTANT
FORMULAS IN ACID-BASE CHEMISTRY.
SO NOW WOULD BE A GREAT TIME TO GRAB SOME HOMEWORK PROBLEMS
AND RE-ENFORCE YOUR UNDERSTANDING.