Jackie L. answered • 05/16/18
Tutor
New to Wyzant
BS in Biochemistry with Acceptance to Medical School
From the chemical formula, we see that it takes 6 molecules of H2O to make 3 molecules of Mg(OH)2. We can simplify this for the sake of the question by saying that 2 molecules of H2O is needed to make 1 molecule of Mg(OH)2. We will keep this in mind for now because we will need this information later on.
The question is asking us how many grams of H2O are needed to make 150g of Mg(OH)2. I’m going to briefly describe our plan for solving this. We start off with 150g of Mg(OH)2. However, since the chemical formula given is based on molecules, we need to convert the grams of Mg(OH)2 to molecules of Mg(OH)2. There is no direct conversion from grams to molecules. We must first convert the grams of Mg(OH)2 to moles and then moles to molecules of Mg(OH)2. The question is asking for grams of H2O though. Using the chemical formula, we use the molecules of Mg(OH)2 to figure out the molecules of H2O (which is where the first paragraph comes into play). Finally, we can convert molecules of H2O to grams of H2O. Like before, we cannot directly convert number of H2O molecules to grams of H2O. We will have to convert number of H2O molecules to moles of H2O to grams of H2O, which will be our final answer. I know it’s a lot of converting units, but with practice, you’ll get faster at answering these problems.
1. Convert grams of Mg(OH)2 to moles of Mg(OH)2.
What value or measurement can we use that connects grams and moles? It’s molar mass. On an exam, the teacher will probably only give you a periodic table with each element’s atomic weight, and you’ll have to add each atom’s weight together to get a molecule’s weight. I’m going to assume you know how to do this, so I won’t show you the steps. The molar mass of 58.32 g/mol. Thus, 150g/58.32g/mol=2.57 mol Mg(OH)2.
The question is asking us how many grams of H2O are needed to make 150g of Mg(OH)2. I’m going to briefly describe our plan for solving this. We start off with 150g of Mg(OH)2. However, since the chemical formula given is based on molecules, we need to convert the grams of Mg(OH)2 to molecules of Mg(OH)2. There is no direct conversion from grams to molecules. We must first convert the grams of Mg(OH)2 to moles and then moles to molecules of Mg(OH)2. The question is asking for grams of H2O though. Using the chemical formula, we use the molecules of Mg(OH)2 to figure out the molecules of H2O (which is where the first paragraph comes into play). Finally, we can convert molecules of H2O to grams of H2O. Like before, we cannot directly convert number of H2O molecules to grams of H2O. We will have to convert number of H2O molecules to moles of H2O to grams of H2O, which will be our final answer. I know it’s a lot of converting units, but with practice, you’ll get faster at answering these problems.
1. Convert grams of Mg(OH)2 to moles of Mg(OH)2.
What value or measurement can we use that connects grams and moles? It’s molar mass. On an exam, the teacher will probably only give you a periodic table with each element’s atomic weight, and you’ll have to add each atom’s weight together to get a molecule’s weight. I’m going to assume you know how to do this, so I won’t show you the steps. The molar mass of 58.32 g/mol. Thus, 150g/58.32g/mol=2.57 mol Mg(OH)2.
2. Convert moles of Mg(OH)2 to molecules of Mg(OH)2.
We will use Avogadro's number (6.022x1023 particles/mol). Particles can mean either atoms or molecules. In our case, it is molecules. Thus, 2.57mol*6.022x1023 molecules/mol=15.49*1023 molecules of Mg(OH)2.
3. Convert molecules of Mg(OH)2 to molecules of H2O.
We will use the information from the first paragraph (1 molecule of Mg(OH)2 requires 2 molecules of H2O). Thus, 15.49*1023 molecules Mg(OH)2*(2 molecules of H2O/1 molecule of Mg(OH)2)=30.98*1023 molecules of H2O.
4. Convert molecules of H2O to moles of H2O.
Same concept as Step 2 except backwards. Thus, 30.98*1023 molecules of H2O*(1 mol H2O/6.022*1023 molecules of H2O)=5.14 mol H2O.
5. Last step! Convert moles of H2O to grams of H2O.
Same concept as Step 1 except backwards. The molar mass of water is 18.01g/mol. Thus, 5.14 mol H2O*(18.01g/mol)=92.6g of H2O, which can be rounded to 93g of H2O.
Your final answer is 93g of H2O.
What you might have noticed is we multiplied Avogadro's number and then divided it, which you might be thinking it was useless. Although that may be true mathematically, it's important to understand how it is used in conversions because the value does not always get cancelled. For this problem, Avogadro's number ended up being cancelled.
Also, you may notice that I always write the molecule I am referring to in the units (e.g. moles, molecules, grams, etc.). When you get more complex problems, it is easy to lose track of what molecule the value you calculate is for. I highly recommend getting into the habit of doing this as it will make your life easier as you progress in chemistry.
Hopefully from my explanation, you were able to figure out where you went wrong. Feel free to respond to my answer if you have any further questions or need any clarifications, such as calculating molar mass!
Jackie
Patience S.
You are an amazing human!!!!12/09/18