Which of the following numbers count as exact numbers and do not count when determining significant figures in a calculation? 1. Density: 3.7 g/mL 2. Volume: 1000 mL/L 3. Length: 2.54 cm/in 4. Speed: 65.2 km/hr 5. Number: 14 aardvarks 6. Mass: 38.741 g

Answer :

(1) Density do not count as exact number.

(2) Volume count as exact number.

(3) Length count as exact number.

(4) Speed do not count as exact number.

(5) Number count as exact number.

(6) Mass do not count as exact number.

Explanation :

Exact numbers : It is a defined or integer value which comes from counting numbers of objects.

They have an infinite number of significant figures.

For example : 12 eggs in a dozen, 1000 g in a kilogram and 2.54 cm in an inch.

Now we have to determine the following numbers count as exact numbers.

(1) Density : 3.7 g/mL

Density do not count as exact number.

(2) Volume : 1000 mL/L

Volume count as exact number.

(3) Length : 2.54 cm/in

Length count as exact number.

(4) Speed : 65.2 km/hr

Speed do not count as exact number.

(5) Number : 14 aardvarks

Number count as exact number.

(6) Mass : 38.741 g

Mass do not count as exact number.

"standard molar enthalpy:

h2o ( liquid water ) : - 285.8 kj/mole

h2o ( water vapor ) : - 241.8 kj / mole.

δhrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0

answer: 44.0 kj"