Sig Fig question - addition and subtraction

I got a question via email:
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Would you help me with the question in the textbook, p.40, Example 1.4 (b)?

1. 4 (b) Subtract 421.23 g from 486 g.

I thought 486's sig fig is 3 and 421.23's is 5. Doesn't the answer have to coincide with the fewest decimal places? The answer was 64.77 so I rounded it up to 64.8, and its sig fig is 3. I don't understand why the correct answer is 65g, its sig fig is 2 then.

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Significant figures are something that takes practice, so let's walk through this one step by step. Before we get too tied up in the math, remember that the whole reason we evaluate significant figures is to help us see where the uncertainty is in a number that we report.

First, let's just "do the math" given in the question... that's the simple part.

486g - 421.23g = 64.77g

Now we get to the more critical part... evaluating the information that we are using to get that answer. For addition and subtraction, we round the result of the addition or subtraction to the least-precise decimal place from the inputs. The "least precise" decimal place tells us where the uncertainty starts.

486g (in this case) really means that the true value is less than 487g but greater than 485g. That's the uncertainty we are trying to keep track of with significant figures. Similarly, 421.23g is really less than 421.24g but greater than 421.22g. When we subtract these values, we round to the "ones" digit because that is the least-precise input.

It's not that unusual to gain or lose sig figs when using the addition & subtraction rules. When adding and subtracting, don't worry as much about counting sig figs, just make sure you're rounding to the correct position in your result. 6.3 and 5.9 each have 2 sig figs, but when you add them together, the result (12.2) has 3 sig figs because we round to the least precise position, in this case the tenths place.

When multiplying and dividing, that's when we count the sig figs. Count the sig figs of the inputs and round the result to the same number of sig figs as the input with the fewest sig figs.

Summer 2020 - Gen Chem Information

Thank you for your interest in General Chemistry during Summer 2020. I am excited to be teaching this course online this summer and look forward to meeting a very diverse array of students! A couple notes and resources to help you all get started:

1. This is a 3-credit, lecture-only course. If you need to take a Gen Chem lab course, you will have to pick that up at another time.

2. There is a pre-requisite math requirement to register for CHEM 150. If you try to register and require an over-ride to get past this hurdle, please send me an email with your DragonID (8-digit number, starts with "14######" or "15########") and a description of the math courses you have taken.

3. The course is asynchronous which means that there are no specific meeting times. I will try to keep you on track during the course, but one of the biggest challenges for most online learners is being responsible for your time management. For the summer courses, I would expect msot students to require 25-35 hours per week to do all the assigned readings, practice, homework, and quizzes.

4. We will be using the Open Educational Resource (OER) textbook from OpenStax, Chemistry 2nd edition. (https://openstax.org/details/books/chemistry-2e) This book is freely available to download as an electronic copy

5. If you prefer a physical textbook, you can get a print copy of the text for a very reasonable cost. The easiest place I have found it is on Amazon (Amazon link). Based upon some of the feedback I see in the comments, it looks like there may be some dissatisfaction with the paperback version of the text, so it might be worth the few extra dollars to get the hardcover. A physical textbook is not required for the course; if you are comfortable using electronic textbooks, the freely available download will serve you perfectly well.

6. Gen Chem I will address chapters 1-9 (approximately) and Gen Chem II will tackle chapters 10-17 & 21.

I think that addresses most of the common questions I've been getting. I hope you are all having a safe and productive Spring, and I look forward to "seeing" all of you this summer.

Preparing for a Final Exam

What's the best way to prepare for a cumulative Final Exam? That can be a little ominous, and the "best" way to study can vary considerably from person to person. At this point, you've (hopefully) found something that works for you, whether that means a quiet corner in the newly remodeled library or a lounge with a little more "white noise". Go with the method that works for you!

With that as a foundation, what should you study? It's probably not a good idea to just sit down with the textbook and start reading on page 1. Use the tools you have available to focus and prioritize your studying time and energy!

1. Start with the exams you've already taken!
Look back at your exams. There are things that you knew very well when you took Exam 1 that have gotten a little foggy over the past few months. The good thing is that you knew that material fairly recently, so it'll probably just take a little review to freshen up those concepts and problems.
What about things you didn't do well on earlier exams? Chemistry (and many other fields...) is a cumulative subject. We tend to look at things from a bunch of different directions, and we often approach a concept or problem 2 or 3 different ways over the course of the semester and year. In the past few months, we might have looked at something differently in a way that suddenly makes perfect sense to you.
Use your exams to jot down and prioritize things to study. It won't be a perfect list, but it will give you a good starting point.

2. Review notes from class
Once you've identified the things you need to review, look back at your notes from class.(Sometimes this serves as a reminder to take better notes in the future!) For some topics, a brief reminder from your notes will be all that you need to bring these concepts and problems up from the cold, dark storage room of your memory. For others, it's a good way to once again identify and prioritize your study topics.

3. Use the book
For topics that are still fuzzy, look up key words in the index to help you find a good place to start. Not sure you remember integrated rate laws? Look it up in the index and hit those pages!
It's also not a bad idea to skim over sections that you're pretty sure you understand. Once you're fairly comfortable with a topic, it becomes easier to pick up some of the more subtle points.
End-of-chapter problems can also be helpful because they present information a little bit differently than I do. I'm not talking "better" or "worse" here, the book just uses different wording to ask the same questions I ask. At the end of my class, I don't just want you to be good at answering questions that I write, I want you to understand chemistry, even if someone else is asking the questions.

If you get through all of those steps and still have time, energy, and a thirst for more knowledge, use your favorite tool to search the World Wide Interwebs Net or stop in and chat with me. This isn't a perfect list of study tips, but it's a good place to start. Identify topics, prioritize your studying time and energy, look for connections and common themes in the material, and good luck. You can do it!

Suggested problems posted

Chapter 5 (and 4, sort of) suggested problems are posted:
http://www.drbodwin.com/teaching/genchem.php

Enjoy. I also tweaked the calendar on that page to reflect the changes we've had.

Welcome to Fall 2013!

Welcome to the first day of Fall 2013 at MSUM!

 If you're looking for the textbook for class, here's the link on Amazon. http://www.amazon.com/Chemistry-Science-Context-Third-Edition/dp/0393934314/ref=sr_1_1?ie=UTF8&qid=1377519676&sr=8-1&keywords=chemistry+3rd+edition+gilbert
This is not an endorsement or recommendation of amazon.com, it's just the quickest way I could find a fairly universal link to the correct textbook. The book is available at the MSUM Bookstore and many other places.