Regents Chemistry Exam August 2022

A sample of helium gas in a sealed, rigid container is at 240. K and 120. kPa. The temperature is increased to 360. K.

55. State the number of significant figures to which the given pressure is expressed. [1]

Rules on how to determine the number of significant figures?

All non-zero digits are significant. This means all numbers: 1,2,3,4,5,6,7,8, and 9 are significant. For example 5.67 has 3 significant figures.

All zeroes in between non-zero numbers are significant. For example zero in 303 is significant.

Leading zeroes (zeroes to the left of a number) are not significant. For example: 0.0009 has only one significant figure (9) since all of the zeroes to the left of it are not significant.

Trailing zeros to the right of a decimal point are significant. For example zeroes in 3.400 are significant.

Terminal zeros in a number without an explicit decimal point may or may not be significant (for example 300). To remove the confusion, we should write out these number using scientific notation (a written in another post)

**Answer:** There are 3 significant figures in 120.

56. Determine the pressure of the helium at 360. K. [1]

We can look at Table T on the Reference table for the combined gas law.

P1V1/T1 = P2V2/T2 The volume is not given and can be assumed to be constant (sealed container) and crossed out, giving us P1/T1 = P2/T2

Let's write out what we are given: P1 = 120.kPa T1 = 240K P2 =? T2= 360K

120.kPa/ 240K = P2/360K

**Answer:** P2= 180. kPa
57. Show a numerical setup for converting 120. kPa to atmospheres. [1]

Table A on the Reference Table states that 101.3 kPa = 1 atm

**Answer:** 120kPa* 1atm/120kPa

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