You must use unit analysis to solve density problems in chemistry (NOT THE FORMULA & ALGEBRA). Remember you are learning the process of unit analysis, not simply finding the correct answer!
Use the technique you have learned in problem solving: what are you looking for, what are you given, put the units together to cancel and solve.
Example #1
A sample of a known metal is dropped into a graduated cylinder with 25.0 mL of water. The level rises to 31.5 mL. If the given density of the metal is 2.56 kg/L, what is the mass of the sample in grams?
Possible Questions
Where did 6.5 mL come from? A substance displaces its volume when immersed in water. The water level rose from 25.0 to 31.5 mL.
Why are there only 2 sig dig in the final answer? When you multiply and divide, you use the least number of sig dig. 2.56 kg/1L has 3, 6.5 mL has 2, and the conversions for L and kg are exact values with an infinite number of sig dig.
Example #2
Where did 6.5 mL come from? A substance displaces its volume when immersed in water. The water level rose from 25.0 to 31.5 mL.
Why are there only 2 sig dig in the final answer? When you multiply and divide, you use the least number of sig dig. 2.56 kg/1L has 3, 6.5 mL has 2, and the conversions for L and kg are exact values with an infinite number of sig dig.
Example #2
What is the volume of a sample of aluminum in L that weighs 56.7 g? The density of aluminum is 2.71 g/mL.
Possible Questions
Why flip the density? To make sure the units of volume will be in the numerator.
I got a different answer (0.153657), why? Remember that 2.71 is in the denominator. You must divide, not multiply. You can either input 56.7 first, divided by 2.72, divided by 1000
OR start with 1 divided by 2.71, times 56.7, divided by 1000.
OR start with 1 divided by 2.71, times 56.7, divided by 1000.
How do we convert gallons to liters on the homework?
ReplyDeleteThere are a list of conversion tables at the back of our text.
ReplyDeletewhen given a problem that begins with 9.23X10^4 - that has 3 sig digs - but during the problem i am convert 1 yard over 36 inches. Is my least amount of sig digs for my answer still going to be 3 because the 1 yard over 36 inches is exact and an infinite number?
ReplyDeleteand if the problem begins with a number like 6.927X10^-2 - that has 4 sig digs - but during the problem i convert 365 days over 1 year - that is an approximation so it only has 3 sig digs....would my answer have only 3 sig digs because that is the least number of sig digs?
ReplyDeleteWhen multiplying, you always use the least # of sig digs. So, in both the above cases, there would only be 3 sig digs in the answer.
ReplyDeleteYes, 36 in per yard, 5280 ft per mile, ... are all exact.
2.54 cm per in, 365 days per year, and 52 wks per year are all approximations. So these use the sig dig rules.
Does the given information always have an infinite number of sig digs? Like if a person has 232 mg of cholesterol per 100 ml of blood, or if the person has 5.2 L of blood. How many sig digs are in those?
ReplyDeleteMeasurements always have sig dig. 232 mg has 3 and 5.2 L has 2.
ReplyDeleteOn number 46 d the problem is as follows: An individual suffering from a high cholesterol level in her blood has 232 mg of cholesterol per 100 mL of blood. If the total blood volume of the individual is 5.2 L, how many grams of total blood cholesterol does the individual's body contain? I converted the mg to g, but where does 5.2 L go in the problem? I keep getting it on the numerator instead of the denominator
ReplyDeleteYou want to end with only grams. g=
ReplyDeleteYou're given the ratio of mg to ml. That is a single given.
Start with 232 mg on top (to make sure you end with g on top), then put 100 mL below it.
On page 32 #30 c, you need to know the volume of the sphere. Can we calculate the volume of the sphere by using the formula given or is there a way to do it using unit analysis?
ReplyDeleteTemperature doesn't affect the density, right?
ReplyDeleteYou can use the formula for a sphere (back of the text) IN unit analysis. Remember that you'll end up with a unit of distanced cubed.
ReplyDeleteYes, temperature does affect density. The temp is constant though, so you can ignore it in this problem.