You are given a 0.6240 g sample of a substance with the generic formula MCl2 ·2 H2 O . After com

You are given a 0.6240 g sample of a substance with the...

Key Concepts

Stoichiometric Calculations

Stoichiometric calculations utilize the relationships dictated by a chemical formula to connect the masses and moles of reactants and products. Here, the stoichiometry of the hydrated salt indicates that each mole of the salt contains a fixed number of water molecules, allowing one to set up equations relating the mass lost upon drying to the molar amounts, ultimately leading to the determination of the unknown metal.

Molar Mass Determination

Molar mass determination involves calculating the mass of one mole of a given substance based on its constituent elements. In this context, after finding the molar mass of the anhydrous salt from experimental data, one subtracts the known contribution of other parts of the molecule (like the chlorine atoms) to identify the atomic mass of the unknown metal element.

Mole Concept

The mole concept is fundamental in chemistry and allows for the conversion between mass and number of particles (atoms, molecules, etc.) using the molar mass as a conversion factor. In problems like this, calculating the moles of water lost or moles of the anhydrous compound is critical to relate experimental mass data to the chemical formula.

Dehydration of Hydrated Salts

This concept involves the removal of water molecules that are integrally bound to a salt in a crystalline structure, resulting in an anhydrous form of the compound. Understanding the dehydration process is important because it enables the determination of the ratio between the anhydrous substance and its water of crystallization, which is essential for stoichiometric calculations.

Transcript

00:01 Let's say we're given 0 .64 grams of some unknown substance with this general formula of mcl2 and 2 moles of water.

00:13 Now this m is going to be our unknown element that we're trying to determine, and it's going to react with two atoms of chlorine and two moles of water.

00:24 And we're given the information that after we dehydrate it, take away the h2o, we're left with 0.

00:31 0 .5471 grams of this mcl2 substance.

00:36 So first, to determine our unknown element, the first thing we're gonna have to do is find out the mass of water that we lost.

00:45 So, if we take this 0 .624, and then we take away the 0 .5471, this should give us the amount of water that we lost in the reaction.

01:02 So we'll do 0 .620 minus 0 .441.

01:08 And that gives us 0 .0769.

01:15 So this is the amount of water that we have here.

01:21 So we're going to divide this number by the molar mass of water to get the number of moles that we have here.

01:32 So 0 .07 69 divided by 18 and that's going to give us 0 .004 all right so this is the amount of moles of water that we have but since we have two here we're going to divide this number by two to set it equal to the one mcl that we have over here.

02:06 So we're going to divide this number by 2.

02:09 And that will give us 0 .0021.

02:19 And then we'll write out a few more decimals just in case.

02:24 So this number, this 0 .0021, that's the number of moles that we have that are represented by each of the elements in our mcl.

02:35 2.

02:37 In order to determine the identity of m, we're going to have to take out the mass of chlorine, and then we're going to subtract that from our dried sample.

02:46 So we take the moles, and we're going to multiply that by the atomic mass of chlorine, which is around 35 .45.

03:02 When you do that, you'll see that the mass is .0757 grams.

03:10 But this is for cl2.

03:16 So we're going to have to multiply this number by 2 because that was just the molar mass of one element of chlorine.

03:24 And that will give us 0 .1514 grams...

Please give Ace some feedback