Dimensional Analysis Help  [ENDORSED]

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Dimensional Analysis Help

Postby BriannaParrington-1B » Fri Apr 06, 2018 1:12 pm

What is the easiest way to perform dimensional analysis on chem problem? How do you know when to do dimensional analysis and when it is not needed?

Jessica Urzua-1H
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Re: Dimensional Analysis Help

Postby Jessica Urzua-1H » Fri Apr 06, 2018 1:20 pm

I would say dimensional analysis is needed in all chemistry problems, unless certain measurements are the same and the question does not specifically ask for one type of measurement. For example, if you have a problem that involves mL/mol instead of L/mol, it is okay to keep your answer in mL/mol as long as the question does not specify that you are looking for liters. I hope this helped in some way!

Chris Qiu 1H
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Re: Dimensional Analysis Help

Postby Chris Qiu 1H » Fri Apr 06, 2018 1:23 pm

You use dimensional analysis whenever you need to change a value from one type of unit (meters) to another unit (kilometers). I find the easiest way to do these calculations is to make sure that after the calculation the only unit(s) that haven't canceled out are your desired units.

Andrew Evans - 1G
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Re: Dimensional Analysis Help

Postby Andrew Evans - 1G » Fri Apr 06, 2018 1:27 pm

Yo. I do dimensional analysis for basically everything on the homework so far. It’s my way of doing what Mr. Dr. Prof. Lavelle said where you use what you know to find what you don’t.

So you start with some value and use any sort of relationships you know to find what you’re looking for.

So I’m just gonna use problem M1 as an example here:
You know the balanced equation (the stoichiometric relationships), and the starting value of NH3. You can use the molar mass of NH3, the molar ratios of NH3/N2H4, and the molar mass of N2H4 to lead you to the theoretical mass product.

—> 35.0 g NH3 (mol NH3 / 17.04 g NH3)(mol N2H4 / 2 mol NH3)(32.06 g N2H4 / mol N2H4) = 32.9 g N2H4

So you mainly use dimensional analysis when making conversions, linking each nominative with the next denominator to cancel out, until you end with the final numerator as your desired unit.

Otherwise, for stuff where you already have a set equation, like for Solution Dilutions (that rhymed), you don’t need to use dimensional analysis. It’s a weird thing figuring out whether you should use it. But yeah. Conversions.

Hope this sorta helped!

–Andrew Evans
Section 1G

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Re: Dimensional Analysis Help

Postby Nina_A_Section1E » Fri Apr 06, 2018 1:30 pm

Use dimensional analysis when you have a quantity in terms of one type of units, but you need to convert it to another type of units.
Some people like to do this 1. step by step, ex. converting g to mol, then a separate equation for mols to atoms, etc.
2. Others like to do this all in 1 step, like below, so you can see all the units canceling out
3,600 s 1 min 1 hour 1 day
_______ x _____ x _______ x ______
1 60s 60 min 24 hours

Megan Phan 1K
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Re: Dimensional Analysis Help  [ENDORSED]

Postby Megan Phan 1K » Fri Apr 06, 2018 2:13 pm

Dimensional analysis is essential throughout all of chemistry. We frequently receive values that can't immediately be applied to the equation problem, unless you convert it to it's proper value. It is then you are able to apply it to your equation/problem and solve. For instance, meters to kilometers (or vice versa), mililiters to liters...so on and so forth. It lays the foundation for you to begin solving.

Jack Martinyan 1L
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Re: Dimensional Analysis Help

Postby Jack Martinyan 1L » Sun Apr 08, 2018 12:00 pm

Dimensional Analysis is a great way to keep track of all your units to make sure that everything crosses out at the end, leaving you with your desired units. The easiest way to perform it is by basically multiplying all your quantities at the same time in one line to see which units cross out. This is a great visual aid.

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