Calculate the mass percent of phosphorus in a strand of DNA that consists of equal amounts of ea

Calculate the mass percent of phosphorus in a strand of DNA...

Key Concepts

Stoichiometry in Mixtures

Stoichiometry involves the quantitative relationships between the amounts of reactants and products in a chemical reaction, and it is also used to calculate the composition of mixtures. In cases where equal amounts of different components, such as the four nitrogenous bases in DNA, are present, stoichiometric calculations ensure that each component's contribution to the overall composition is accurately accounted for.

Mass Percent Composition

Mass percent composition is a method used in chemistry to determine the proportion of a particular element within a compound. It is calculated by dividing the mass of the element by the total molecular mass of the compound and then multiplying by 100. This calculation is essential for understanding the relative abundance of elements in molecules.

Phosphorus in Biochemical Molecules

Phosphorus is a key element in many biological molecules and is especially important in the structure of DNA. In DNA, phosphorus is found in the phosphate groups that interconnect the deoxyribose sugars, contributing to the stability and negative charge of the genetic material.

DNA Nucleotide Structure

DNA is composed of repeating units called nucleotides, each consisting of a deoxyribose sugar, a phosphate group, and a nitrogenous base. The phosphate group, which contains phosphorus, is an integral part of the DNA backbone and links the sugar molecules of adjacent nucleotides through phosphodiester bonds.

Transcript

00:02 In problem 82, we need to find the mass percentage of phosphorus in dna.

00:31 We are given the following information.

00:37 We have equal amounts of each nucleotide.

00:55 So basically, we could have something that looks like this.

01:04 The diamonds are phosphate groups, circles or sugars, in particular, the 5 -deoxyribos, that attached are the bases, adenine, thymine, etc.

01:42 Cytocene and guanine.

01:44 Let's assume that we have a section of this that repeats itself.

01:51 The order doesn't matter because all we're doing is calculating formula weights, no matter what order the bases are in.

02:06 So what we can do is calculate the formula weight of, the repeating unit, and we can treat that as formula weight of dna, and just divide, you know, four phosphorus atomic masses by the formula weight of the repeating unit.

02:47 And that will give us our mass percentage of phosphorus and dna.

02:57 So what we need to do is find the formula weights of these units.

03:03 And to do that, i'm going to draw a diagram, a more detailed diagram.

03:08 Of what at least a section of this unit is going to look like.

03:16 So what we have is our phosphate group.

03:32 Then we're going to have our sugar, which looks like this, the five -membered ring with oxygen in it.

03:46 We draw the other stuff, hydrogens and two hydrogens here, one there.

04:02 This is where the base attaches.

04:07 We'll look at that separately.

04:08 And down here on the sugar is an oxygen that connects to the next phosphate unit.

04:20 So we kind of have something that looks like this.

04:24 We have a big long line of these, a phosphate, a sugar, and a base.

04:30 We're going to calculate the 4mm weight of each of these.

04:33 For the phosphate, i'm just going to use the phosphorus in three oxygens.

04:46 For the sugar, i'm going to use this part.

04:54 And then the base, including its nitrogen that bonds here, will be calculated separately.

05:06 Now i'm not going to calculate all of these, but we'll do some sample calculations for the formula weights.

05:14 For example, the phosphate unit is going to be po3.

05:21 So we have one phosphorus, which is 30 .97 amu or grams per mole.

05:30 And we add that to three oxygens.

05:37 Each oxygen has a weight of 16.

05:42 We've got three of those, multiply it by three, and add it to 30 .97 for the phosphorus.

05:50 We get a total of 78 .97 grams per mole for each phosphate group.

06:02 This is the formula weight for that part.

06:08 For the sugar, we have a five -member ring, four of those.

06:18 Those are carbons, but we have another carbon up here, so c5.

06:24 We got two, three, four, five, six, seven hydrogens and two oxygens.

06:38 So we add those together.

06:40 We got five times 12 .01 for carbon, plus seven times one.

06:48 1 .008 for hydrogen plus 2 times 16 for oxygen.

06:56 When you add those all together, multiply first, and then add, you get a formula weight of 99 .106.

07:12 Now for the base, we got four different bases to choose from.

07:16 I'm only going to do one as a sample calculation.

07:19 So i chose adenine.

07:27 So let's look at that...

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