Early tables of atomic weights (masses) were generated by...

Early tables of atomic weights (masses) were generated by measuring the mass of a substance that reacts with $1.00 \mathrm{g}$ of oxygen. Given the following data and taking the atomic mass of hydrogen as $1.00,$ generate a table of relative atomic masses for oxygen, sodium, and magnesium.

Key Concepts

Relative Atomic Mass

This is a comparison of an atom's mass to that of another, typically using hydrogen as the standard with a value of 1. It allows chemists to express atomic weights on a unified scale, facilitating the comparison and computation of element proportions in chemical reactions.

Stoichiometry

Stoichiometry involves the calculation of reactants and products in chemical reactions based on the balanced chemical equations. It is fundamental in determining how different masses of elements react according to fixed ratio principles, enabling the derivation of relative atomic masses.

Law of Definite Proportions

Also known as the law of constant composition, this principle states that a chemical compound is always composed of the same elements in the same proportion by mass. This foundational concept supports the method of using fixed mass ratios of reacting substances to determine atomic weights.

Transcript

00:02 Chapter 1, problem 35, initially looks like a pretty tricky problem, but we can work through it to get to the correct answer.

00:11 So here we're given a table of elements, mass that combines with one gram of oxygen, and the assumed formula.

00:19 We're asked to use these values to calculate the relative atomic masses for oxygen, sodium, and magnesium.

00:27 So in these we will use the same idea of the proportion for each of our calculations.

00:34 The proportion is as follows, and let's use ho, the hydrogen elements, for our example.

00:43 If we have the mass of oxygen that we're using in our combination over the mass of hydrogen, since we have a one -to -one or one atom of hydrogen is bonding with one atom of oxygen, we can say that this ratio is the same as the atomic mass, or the am of oxygen, over the am of hydrogen.

01:15 The equation gives us the mass of oxygen and hydrogen, and tells us to assume the atomic mass of hydrogen as what.

01:25 Using those three values, we can then calculate what we need, the atomic mass of oxygen.

01:31 So let's go ahead and try this out for the first problem.

01:34 We're told that the mass of oxygen is the same for all of them, one gram.

01:40 The table tells us that the mass of hydrogen is 0 .126 grams.

01:49 We're solving for the atomic mass of oxygen.

01:53 And the problem also tells us the atomic mass of hydrogen is one.

01:59 So if we calculate for the atomic mass of oxygen, we get 7 .9 grams.

02:06 Now, if you're paying careful attention, you'll notice that this is incorrect.

02:12 The true atomic mass of oxygen is 16.

02:16 But notice that the assumed formula here is h0, a 1 -to -1 ratio.

02:21 When we know, in fact, when oxygen and hydrogen combined, we get h -2o.