Chemistry question: Mole Calculation?

Question

My teacher wants me to be able to convert moles to grams and grams to moles using dimensional analysis. Here are two examples

1: How many grams are in 88.1 moles of magnesium?
2: How many moles are in 2.3 grams of phosphorus?

The answers are not as important as the process by which you get them... Please help explaining how I solve these because I don't understand dimensional analysis and I feel hopeless with this homework.

Thanks for any helpers! :D

Answer 1

moles= mass/Molar mass



1. 88.1 moles of magnesium, which has a Mr of 24.3



so you would do moles x Mr

88.1x 24.3 = 2140.83g



2. 2.3g of phosphorus, which has a Mr of 31.0



so you would do mass/Mr

2.3/31.0= 0.074 (2dp)



You can place the moles= mass/mr into a triange to help you solve it here's a link to what it would look like:

http://www.google.co.uk/imgres?q=moles+triangle&um=1&hl=en&sa=N&biw=1366&bih=667&tbm=isch&tbnid=c81KlAcgB0U4PM:&imgrefurl=http://perpettualy15.wordpress.com/2007/12/27/moles-an-easy-undestranding/&docid=-tOzShMbpKT_JM&imgurl=https://wiki.brown.edu/confluence/download/attachments/431/mole.jpg&w=219&h=185&ei=ngXQTuaoGMKo8QPYsMz7Dw&zoom=1&iact=hc&vpx=210&vpy=133&dur=4561&hovh=148&hovw=175&tx=131&ty=137&sig=101845267273756905779&page=1&tbnh=130&tbnw=154&start=0&ndsp=20&ved=1t:429,r:0,s:0

Answer 2

Dimensional analysis means that you arrive at the answer by following units rather than formulas.

For example, the first question asks you to find the mass in grams, so you know that your final answer must have units of gram. You are given a number that carries units of moles; to arrive at an answer with units of gram only, you need to multiply the 88.1 mol by a number that has units of gram/mol, so that the mol cancel out and you're left with gram.



The only number that has units of gram/mol is the molecular mass of magnesium.

So, 88.1 mol x (24.31 gram/mol) = 2141.711 grams



2) Follow the same logic that you used for number 1. However, this time you want the grams to cancel out leaving you with moles only.

2.3 grams x (mol/30.974 grams) = 0.0743 moles

Answer 3

if you're dealing with debris, molecules, ions, atoms, or the like, the use avagodro's variety (6.02x10^23/mol). it is used to rework into moles. To volume (only for gases at STP), that is 22.4L/mol. For mass, use the molar mass. .340 mol NaC2H3O2 is more advantageous by utilising the molar mass divided by utilising a million mol. .340 mol NaC2H3O2* (80 2.04 g/a million mol)=27.9 g NaC2H3O2 If the question change into what percentage moles in .340 g of NaC2H3O2, it would want to be the moles divided by utilising the molar mass. that is because of the guidelines for algebra- as a thanks to cancel a gadgets signal, you ought to have it once interior the numerator and once interior the denominator. .340 g NaC2H3O2* (a million mol/80 2.04 g)= .00414 mol NaC2H3O2 For molecules/atoms/ions/etc, shall we are saying you've been given .340 g NaC2H3O2 and were requested many molecules there are. First, do as above. From there, convert moles into molecules. .340 g NaC2H3O2* (a million mol/80 2.04 g)*(6.02x10^23/a million mol)=2.49x10^21 molecules NaC2H3O2 wish this enables!