Section 1.5—Significant Digits

Section 1.5 A Counting significant digits

Taking & Using Measurements You learned in Section 1.3 how to take careful measurements Most of the time, you will need to complete calculations with those measurements to understand your results 1.00 g 3.0 mL = g/mL If the actual measurements were only taken to 1 or 2 decimal places… how can the answer be known to and infinite number of decimal places? It can’t!

Significant Digits A significant digit is anything that you measured in the lab—it has physical meaning The real purpose of “significant digits” is to know how many places to record in an answer from a calculation But before we can do this, we need to learn how to count significant digits in a measurement

Significant Digit Rules 1 All measured numbers are significant 2 All non-zero numbers are significant 3 Middle zeros are always significant 4 Trailing zeros are significant if there’s a decimal place 5 Leading zeros are never significant

All the fuss about zeros g Middle zeros are important…we know that’s a zero (as opposed to being 112.5)…it was measured to be a zero mL The convention is that if there are ending zeros with a decimal place, the zeros were measured and it’s indicating how precise the measurement was is between and is between 124 and m The leading zeros will dissapear if the units are changed without affecting the physical meaning or precision…therefore they are not significant m is the same as 127 mm

Sum it up into 2 Rules 1 If there is no decimal point in the number, count from the first non-zero number to the last non-zero number 2 If there is a decimal point (anywhere in the number), count from the first non-zero number to the very end The 4 earlier rules can be summed up into 2 general rules

Examples of Summary Rule 1 Example: Count the number of significant figures in each number If there is no decimal point in the number, count from the first non-zero number to the last non-zero number

Examples of Summary Rule 1 Example: Count the number of significant figures in each number If there is no decimal point in the number, count from the first non-zero number to the last non-zero number 3 significant digits 4 significant digits 1 significant digit 2 significant digits

Examples of Summary Rule 2 Example: Count the number of significant figures in each number If there is a decimal point (anywhere in the number), count from the first non-zero number to the very end

Examples of Summary Rule 2 Example: Count the number of significant figures in each number significant digits 4 significant digits 2 If there is a decimal point (anywhere in the number), count from the first non-zero number to the very end

Importance of Trailing Zeros Just because the zero isn’t “significant” doesn’t mean it’s not important and you don’t have to write it! “250 m” is not the same thing as “25 m” just because the zero isn’t significant The zero not being significant just tells us that it’s a broader range…the real value of “250 m” is between 240 m & 260 m. “250. m” with the zero being significant tells us the range is from 249 m to 251 m

Let’s Practice Example: Count the number of significant figures in each number 1020 m g m mL g

Let’s Practice Example: Count the number of significant figures in each number 1020 m g m mL g 3 significant digits 4 significant digits 5 significant digits

Section 1.5 B Calculations with significant digits

Performing Calculations with Sig Digs 1 Addition & Subtraction: Answer has least number of decimal places as appears in the problem 2 Multiplication & Division: Answer has least number of significant figures as appears in the problem When recording a calculated answer, you can only be as precise as your least precise measurement Always complete the calculations first, and then round at the end!

Addition & Subtraction Example #1 Example: Compute & write the answer with the correct number of sig digs g g This answer assumes the missing digit in the problem is a zero…but we really don’t have any idea what it is 1 Addition & Subtraction: Answer has least number of decimal places as appears in the problem g

Addition & Subtraction Example #1 Example: Compute & write the answer with the correct number of sig digs g g 1 Addition & Subtraction: Answer has least number of decimal places as appears in the problem g g 3 decimal places 2 decimal places Lowest is “2” Answer is rounded to 2 decimal places

Addition & Subtraction Example #2 Example: Compute & write the answer with the correct number of sig digs mL mL This answer assumes the missing digit in the problem is a zero…but we really don’t have any idea what it is 1 Addition & Subtraction: Answer has least number of decimal places as appears in the problem mL

Addition & Subtraction Example #2 Example: Compute & write the answer with the correct number of sig digs mL mL 1 Addition & Subtraction: Answer has least number of decimal places as appears in the problem 8.01 mL 2 decimal places 3 decimal places Lowest is “2” Answer is rounded to 2 decimal places mL

Multiplication & Division Example #1 Example: Compute & write the answer with the correct number of sig digs g 2.7 mL = g/mL 2 Multiplication & Division: Answer has least number of significant figures as appears in the problem

Multiplication & Division Example #1 Example: Compute & write the answer with the correct number of sig digs 3.8 g/mL 4 significant digits 2 significant digits Lowest is “2” Answer is rounded to 2 sig digs g 2.7 mL = g/mL 2 Multiplication & Division: Answer has least number of significant figures as appears in the problem

Multiplication & Division Example #2 Example: Compute & write the answer with the correct number of sig digs g/mL  2.75 mL g 2 Multiplication & Division: Answer has least number of significant figures as appears in the problem

Multiplication & Division Example #2 Example: Compute & write the answer with the correct number of sig digs 4.69 g 4 significant dig 3 significant dig Lowest is “3” Answer is rounded to 3 significant digits 2 Multiplication & Division: Answer has least number of significant figures as appears in the problem g/mL  2.75 mL g

Let’s Practice #1 Example: Compute & write the answer with the correct number of sig digs g g

Let’s Practice #1 Example: Compute & write the answer with the correct number of sig digs 1.2 g 3 decimal places 1 decimal place Lowest is “1” Answer is rounded to 1 decimal place g Addition & Subtraction use number of decimal places! g g

Let’s Practice #2 Example: Compute & write the answer with the correct number of sig digs 2.5 g/mL  23.5 mL

Let’s Practice #2 Example: Compute & write the answer with the correct number of sig digs 59 g 2 significant dig 3 significant dig Lowest is “2” Answer is rounded to 2 significant digits 2.5 g/mL  23.5 mL g Multiplication & Division use number of significant digits!

Let’s Practice #3 Example: Compute & write the answer with the correct number of sig digs g 2.34 mL

Let’s Practice #3 Example: Compute & write the answer with the correct number of sig digs g/mL 4 significant digits 3 significant digits Lowest is “3” Answer is rounded to 3 sig digs g 2.34 mL = g/mL Multiplication & Division use number of significant digits!