Determination Of Atomic Weight (1959)

[Music] when you first began the study of chemistry you learn that the chemists have assigned to each element a characteristic atomic weight now the determination of these Atomic weights is one of the basic processes in chemistry because the atomic weights of the elements are used in a great variety of calculations uh in chemistry today we're going to demonstrate how the atomic weight of an element can be determined uh using the law of dong and petite and some other relationships uh in chemistry dong and petit law deals with the specific heat of the element now specific heat is defined as the number of calories required to change the temperature of 1 gram of the substance 1° centigrade and this is a quantity which is going to be possible for us to determine in the layup knowing the specific heat we can calculate the approximate atomic weight because dong and petite found that when the constant 6 4 was divided by the specific heat the result was approximately equal to the atomic weight for most elements you previously discovered that the equivalent weight multiplied by the veence or the combining capacity equals the atomic weight and you also know that the veilance of an element is always a whole number now the equivalent weight is another quantity which is possible for us to determine in the lab rather easily so using these two relationships it's actually possible for us to calculate the exact atomic weight first we'll calculate the approximate atomic weight using the specific heat method then we'll calculate the equivalent weight exactly then we'll determine which whole number the veence we have to multiply our equivalent weight by to get a value which is approximately equal to our atomic weight and that number will be the exact atomic weight of the element our first step then will be to calculate the approximate atomic weight using the dong and petite relationship and this will require us first of all to calculate the specific heat of the element which we will now proceed to do the first step in this method for determining the atomic weight of Copper is to determine the specific heat of copper I have a small block of copper metal here which has been placed on the pan of the balance we're determining the weight of this piece of copper uh on a rather inaccurate balance but great accuracy is not necessary at this stage of the proceedings because our determination of the approximate atomic weight of Copper is just that approximate and great care and weighing would be a waste of time so the weight of this piece of copper has rather rapidly been found to be 39.2 G this weight should be recorded because it will be used later we must now heat the sample of copper to a fairly high temperature and then also measure that temperature accurately we've tied a piece of string around the copper so that we can lower it into a test tube you'll notice that the top of the copper block has a small hole drilled in it which permits us to insert a thermometer uh into the copper block we then place the copper in the test tube slide it to the bottom with the string place the test tube in the beaker of water which we're heating with the bunson burner and clamp it in place we'll then place the thermometer with the bulb of the thermometer sticking into the copper block we'll now boil the water and wait until the temperature of the copper becomes constant while the copper is heating we'll prepare the next piece of apparatus uh for the rest of the experiment in this graduated cylinder I've measured out exactly 35 mL of water We'll add this to the small Beaker a thermometer which is graduated in fifth of a degree has been mounted in the stand and we will determine the temperature of the water before the copper has been added to it the temperature of the water in the small Beaker is 33.6 De temperature of the copper metal is 99° cenr will now remove the thermometer and add the piece of copper to the small Beaker temperature of the water in the beaker is rising and we will read the highest temperature reached by the mixture of copper and water the final temperature reached by the mixture is 39.9 De centigrade we can now proceed to calculate the specific heat of the copper uh from the data which uh We've obtained in this experiment we found that our little sample of copper weighed 39.2 G we then placed this copper in the test tube immersed the test tube in the beaker of boiling water and found that the final temperature reached by the copper was 99° Centigrade we then plac this copper in a beaker containing 35 M which we will assume to be 35 G of water we note the temperature of this water before we placed the copper in it was 33.6 de Centigrade now the final temperature of both the copper and the water was 39.9 de cenr therefore the change in temperature of the water was 6.3 de this is from 33.6 up to 39 .9 while the change in temperature of the copper is 59.1 De this is the 99° down to 39.9 De and with this information we can calculate the specific heat of the copper because the specific heat of the copper is equal to the grams of water multiplied by the temperature change of the water divided by the gram of copper multiplied by the temperature change of the copper and we have all four of these figures this then becomes 35 G of water * 6.3 de over 39.2 G of copper * 59.1 de and carrying out the multiplication and division we find that the specific heat of the copper is determined experimentally to be about 095 calories per gram per degree centigrade and we will use this figure now in calculating the approximate atomic weight of coer you'll recall that the dong petite relationship is that the approximate atomic weight equals 6.4 ided the specific [Applause] heat now we've determined the specific heat experimentally for copper our value being 095 and when this computation is carried out we find that the approximate atomic weight of Copper is 67 4 it's difficult to say how accurate this figure is but uh it's probably plus or minus 5 uh or so as an order of magnitude this figure will be very useful to us later on in determining the veence of copper the equivalent weight of an element is defined as the number of grams of the element that will combine with or displace 8 G of oxygen now we're going to determine the equivalent weight of Copper starting with copper oxide copper oxide which I have some in this bottle is a black powder this is a very finely divided sample here we're going to weigh out a carefully measured quantity of copper oxide and then we're going to remove the oxygen from its combination with copper oxide leaving metallic copper and by subtract ing the weight of the Copper from the weight of the copper oxide we'll be able to tell how much copper combined with how much oxygen and from that data calculate the equivalent weight of Copper oxide now the small porcelain boat on the left hand pan of the balance has previously been carefully weighed its weight is 6. 6475 g 66475 g this figure should be recorded as the weight of the empty boat now to this boat I've added uh some copper oxide and the boat plus the copper oxide is now being reweighed this value is 8.81 120 g 8.81 120 that's the boat plus the copper oxide so that the particular sample of copper oxide that we're using the black powder in the boat weighs 21645 G and this figure will be of importance to us later on in our calculation our sample of uh copper oxide in the porcelain boat has been carefully placed in the end of the test tube here we have a uh hydrogen generator set up with Mossy zinc in the ear Meer flask to which we've added some water we're going to add some hydrochloric acid to this flask this will cause the generation of hydrogen we'll add more acid from time to time to keep a steady flow of hydrogen coming from the generator now the hydrogen is passing through this tube through the calcium chloride drying tube which will pick up any water vapor or spray and then through this tube to the rear of the test tube there it will flow over the copper oxide and back out of this exit tube now we'll permit the generator to run for several minutes to displace all of the air in the system we'll also wrap the generator bottle with a towel to prevent the flying around of glass fragments if the generator should explode during the course of the experiment we're now heating the copper oxide in the tube in a stream of hydrogen the hydrogen is coming out of this glass tube we're looking down into the reaction from the top you'll notice that at the closed end of the test tube most of the copper oxide in the boat has already been converted to metallic copper you'll also notice that considerable quantities of steam have condensed into water here in the cool part of the tube this water is formed in the reaction between the copper oxide and the hydrogen in a few moments all of the copper oxide will have been reduced to Pure copper and we'll then let the test tube cool in an atmosphere of hydrogen to avoid any possibility of any reoxidation due to oxygen in the air after the reaction is completed the boat should contain only metallic copper formed in this reaction we've now completed the weighing of our sample of metallic copper in the boat all of the copper oxide has been converted to Copper we've cooled the sample weigh it and found that the boat plus the sample now weighs 8.37 [Music] 75 G 8. 3775 and with this information we can now proceed to calculate the equivalent weight the veence and the exact atomic weight of Copper we found that our empty porcelain boat weighed 66475 G when we added the copper oxide to the boat the two together then weighed 8.81 12 G by subtracting this figure from this one we find that the weight of the copper oxide is 21645 G then we placed the bat in the tube and heated it in a current of hydrogen and we found that the Bol after the reaction containing the copper weighed 8. 3776 G now from this figure we should subtract the weight of the boat to obtain the weight of the copper left and this is 17301 G now if this is the weight of the copper and we added 21645 G of copper oxide by subtracting this figure from this one we obtain the weight of the oxygen combined with the copper or 4344 G Now using these two figures the weight of the copper and the weight of the oxygen we may calculate a rather accurate equivalent weight for copper because we know that 17301 g of copper was combined with 4344 G of oxygen and the equivalent weight is defined as the number of grams of copper combined with 8 G of oxygen so by using a rather simple proportion there we calculate the equivalent weight of Copper and find it to be 31.8 G this then is the value for the equivalent weight earlier we saw that the equivalent weight times the veence equals the atomic [Applause] weight we've calculated the equivalent weight of Copper from this experiment our value was 3186 G we've also calculated an approximate atomic weight using the dong and petite method and this approximate atomic weight [Applause] 67.4 if we multiply 31.8 6 by the veence in we ought to get something approximating 67.4 we can solve this relationship for the veence and we get 2.11 now we know that the veilance is always a whole number and this calculation tells us that the veence of copper in this compound then is two now we have to return to our original formula we still know the accurate equivalent weight is 31.8 6 G we know now that the veilance is two multiplying these two accurately determined figures together uh should give us an accurate value for the atomic weight this multiplication when carried out you 63.7 2 as our value for the gram atomic weight of Copper now this experiment uh was not done with the utmost possible precision and as a result we have as usual introduced a small error by Consulting a Table of atomic weight we find that the recognized value for the atomic weight of Copper is 63.5 7 our determination then is a good check on the accepted value differing by only .15 and this gives us an error of 0.15 ided 63.5 7 time 100 of 210 of 1% 23% is our error and considering the inaccuracies attendant to this particular method uh this is certainly not an excessive error we've seen then how chemist proceeds to calculate the atomic weight of an element from data which is readily available in the lab he can get an approximate atomic weight using the dong and petite relation ship he can get an accurate equivalent weight by a method such as we employed today the reduction of copper oxide he can then determine a veillance by using the two figures just mentioned and with the veillance and an accurate equivalent weight he can calculate an accurate atomic weight [Music]