What is the mass of 5.2 moles of NaF?

Answer 1

Let’s use dimensional analysis

5.2 mol NaF * (41.99g NaF)/ (1 mol NaF)= 220 g NaF (2 significant figures)

Answer 2

A mole of any substance is count of the substance that equals Avogadro's number. This question is asking first what is the mass of an Avogadro's number of NaF molecules. The periodic table contains listing for the mass of an Avagadro's number of atoms for each element. The mass for an Avagadro's number of carbon isotope 12 is 12 grams. The periodic table mass values listed are the masses for the weighted averages of the natural presence of the isoptopes for a given element. Carbon exists as C12 ( 6 protons and 6 neutrons), C-14 (6 protons and 8 neutrons), etc.

Second, this question only provides the count of NaF atoms to two significant figures-the associated mass for this count only needs to have an accuracy to the same number of significant figures (2). The calculation for the mass for one mole of NaF then only requires summing the mass for one mole of Na and one mole of F expressed to only two significant figures.

Elemental Sodium (Na) has an experimental molar mass of 22.99 grams ( 11 protons and 12 neutrons on average)

Elemental Fluorine (F) has an experimental molar mass of 19.00 grams ( 9 protons and 10 neutrons)

The Sodium cation has lost the mass of an electron and the Fluoride anion has gained that same mass. Therefore the atom NaF to two sig figures has a molar mass of 23 +19 = 42 grams.

So in the laboratory, if my balance indicates 42 grams of material is present on the weighing dish and the the chemical formula for this substance is NaF; the count of molecules of NaF in that dish is 1.0 of Avagadro's number and accurate in that count to 2 significant figures.

Now suppose we want to scale up our count to 5.2 times an Avagadro's number or 5.2 times a mole of molecules of NaF. All one need do is multiply the mass of one mole by this scaling factor and report the result to two significant figures.

42 grams scaled by 5.2 to two significant figures is 220 grams or 2.2 x10^2 grams or 0.22 Kg

A little aside on significant figures; this question asks for a conversion in units for 5.2 of another unit or 0.52 x 10 ^1, on writing this in terms of a mantissa and exponent one can see that the measure of the units provided only have a significance to two figures. If instead the question asked for conversion of 5,20 units, we can write this as 0.520X10^1 with no loss of information and see that the mantissa has three places and thus the original information is significant in measure to three figures.

The Avagadro constant is defined as the number of atoms in 12 grams of the isotope ¹²C), To a first approximation; 6×10²³ times its molecular weight. The exact vale published by the System Internationale is exactly 6.02214076×10²³ mol⁻¹