Стехіометричні обчислення з густиною

Stoichiometry Using Density

We've been studying stoichiometry. It's the interrelationship between different quantities of reactants and products. The basis of stoichiometry is dimensional analysis. We are converting from one unit to another. So far, we used the molar mass and mole ratios as our conversion factors. We've investigated mole-to-mole and mass-to-mass conversions.

But, stoichiometry can provide us with so much more information than just mass or moles! Stoichiometry is a way of using what you know, and applying it, to figure out what you don’t know. Practically, we've determined the limiting reagent and calculated the theoretical yield for various reactions. Eventually, we'll compare the theoretical yield and the actual yield. Why? So we can calculate our error as a percent yield. But, in this lesson, we'll look at a unique twist on stoichiometry problems.

So far, most of our reactions have involved gases. But, sometimes we are reacting liquids or solutions. For them, we may want to determine the volume. How do we do it? Within stoichiometry, density allows us to convert our units from mass to volume or vice versa.

Let's look at an example! Let's say we’re given the acid-base neutralization reaction between sodium hydroxide and hydrochloric acid. Remember, it's a double-replacement reaction. So, the sodium and chloride ions will pair up to form sodium chloride. The hydroxide ion will partner with the hydrogen ion to produce water.

$NaOH_{(aq)} + HCl_{(aq)} \rightarrow H_{2}O_{(l)} + NaCl_{(aq)}$

What's a potential question we could be asked? What is the volume of water (in mL) made from the neutralization of 2.0 g HCl with NaOH? The density of water is 1.0 g/mL.

In the photo above, we start out with what we know. We have 2.0 grams of $HCl$. We also know that we want to convert the amount of a starting material to the amount of one of our products - water. The mole is what allows us to do this. How? By utilizing our chemical equation’s mole ratio. Our equation is balanced as is. Therefore, the mole ratio of $NaOH$ to $H_{2}O$ is 1:1. Now, how do we go from moles of $H_{2}O$ to mass? Using the periodic table, we find that water's molar mass is $18.02\frac{g}{mol}$. From here, we use density to go from mass to volume.

Ultimate Takeaway:

In stoichiometry, density allows us to convert from mass to volume or vice versa.

(Video) Stoichiometry with Density

by Chem Help ASAP

In this video, Chem Help ASAP explains how to use density in stoichiometry problems. You'll still need to interconvert moles and masses. But, sometimes, you're given a liquid. So, you'll need to find the volume. For that, you use density! This video will show you how.