What is the percent composition of each element in MgCl2 MgCl2 the molar masses of Mg and Cl are 25.53 and 25.52 g mol respectively?

Answer

Hint: The formula unit mass is the sum of the atomic masses of all the elements present in the corresponding compound. Now, the chemical formula of magnesium chloride is MgCl$_2$. Complete step by step answer:Firstly, let us define the formula unit mass. The formula unit mass can be defined as the sum of atomic masses of all atoms in the compound, and the formula unit is used for the substances made up of ions.Now, we are given with the compound i.e. magnesium chloride having the chemical formula MgCl$_2$. The atomic mass of magnesium is 24 u, whereas chlorine has the atomic mass 35.5 u.Now, the formula unit mass of magnesium chloride is equal to atomic mass of magnesium added with the atomic mass of two units of chlorine. It can be represented as: formula unit mass of MgCl$_2$ = atomic mass of Mg + 2$\times$atomic mass of ClAfter substituting the value of masses, we get formula unit mass = 24 u+2$\times$35.5 u=95 uTherefore, the formula unit mass of magnesium chloride is 95 u. The correct option is B.

Note: Don’t get confused between the atomic mass unit and formula unit mass. While calculating the formula unit mass we multiplied the atomic mass of chlorine by 2 as in the chemical formula we can see there are 2 units of chlorine.

Molar mass of MgCl2 = 95.211 g/mol

This compound is also known as Magnesium Chloride.

Convert grams MgCl2 to moles  or  moles MgCl2 to grams

Molecular weight calculation:
24.305 + 35.453*2

›› Percent composition by element

›› Calculate the molecular weight of a chemical compound

›› More information on molar mass and molecular weight

In chemistry, the formula weight is a quantity computed by multiplying the atomic weight (in atomic mass units) of each element in a chemical formula by the number of atoms of that element present in the formula, then adding all of these products together.

The atomic weights used on this site come from NIST, the National Institute of Standards and Technology. We use the most common isotopes. This is how to calculate molar mass (average molecular weight), which is based on isotropically weighted averages. This is not the same as molecular mass, which is the mass of a single molecule of well-defined isotopes. For bulk stoichiometric calculations, we are usually determining molar mass, which may also be called standard atomic weight or average atomic mass.

If the formula used in calculating molar mass is the molecular formula, the formula weight computed is the molecular weight. The percentage by weight of any atom or group of atoms in a compound can be computed by dividing the total weight of the atom (or group of atoms) in the formula by the formula weight and multiplying by 100.

A common request on this site is to convert grams to moles. To complete this calculation, you have to know what substance you are trying to convert. The reason is that the molar mass of the substance affects the conversion. This site explains how to find molar mass.

Finding molar mass starts with units of grams per mole (g/mol). When calculating molecular weight of a chemical compound, it tells us how many grams are in one mole of that substance. The formula weight is simply the weight in atomic mass units of all the atoms in a given formula.

Using the chemical formula of the compound and the periodic table of elements, we can add up the atomic weights and calculate molecular weight of the substance.

Formula weights are especially useful in determining the relative weights of reagents and products in a chemical reaction. These relative weights computed from the chemical equation are sometimes called equation weights.

Percent composition is the percentage by mass of each element in a compound. first step to finding percent composition is calculating the formula/molecular weight or the molar mass of the compound you want to know the percent composition of. Formula weight is the sum of atomic masses of the atoms in the Ionic compound. Molecular weight is the sum of atomic masses of the atoms in a molecule. Both weights are Identical to the molar mass of a substance in amu’s or grams as ratios are similar. For example MgCl2:

Magnesium: (1)24.305

Chlorine : + (2) 35.45= 70.9

Magnesium Chloride=  95.205 g/mol or amu

Now that the mass is found you divide the part by the whole multiplied by a hundred to find the percentage. In this case you want to find the percentage of Magnesium in Magnesium Chloride.

Magnesium/Magnesium Chloride = 24.305g Mg/95.205 MgCl2 x 100 = 25.53%

Another example:

https://www.delsearegional.us/Academic/Classes/highschool/science/chemistry/firstyear/Notes/academic/Unit03/06percent.html

Finding the mass of an element in a substance is very similar to the finding percent composition. You just put the substance’s mass over one and multiply it by the molar mass of the element divided by the molar mass of the whole formula. For example 24 grams of MgCl2 and you want to find how much magnesium and chlorine make up the mass of the compound.

24.00g/1 x 70.9g of 1 mole of Cl2/95.205 molar mass of MgCl2 = 17.873grams of Chlorine

24.00/1 x  24.305 of 1 mole of Mg/95.205 molar mass of MgCl2 = 6.126 grams of Magnesium

Adding up the two masses together you get 23.999g which is only off due to rounding, but that is how you calculate how much mass of each element is in a substance.

Just as one can calculate percents from formulas you can also calculate formulas from percents. These are called empirical formulas and Molecular formulas. Molecular formulas are all about finding molar ratios between atoms. In Molecular formula equations if percentages are only given for each element assume the sample is a hundred grams. For example 50%C and 50%O would be 50 grams oxygen and 50 grams Carbon. First step of finding Molecular formulas is to divide the mass of the element given by one. Secondly multiply the mass of the single element by 1 mole divided by that elements molar number. Repeat this process for all elements, then divide all products by the smallest product to get molar numbers. If molar numbers such as 0.5, 0.4, 0.3, 0.7 or 0.6 are given multiply all numbers by two in till numbers are close to a whole number. The empirical formula is the simplified version of the molecular formula. So if the molecular formula is C6H12O6 then the empirical formula would be CH2O. For example 10 grams carbon, 15 grams nitrogen, and 20 grams of oxygen are given.

10g of C/1 x 1mole/12.011g= 0.8325 C/0.8325 = 1 x 4 = 4 mole

15g of N/1 x 1mole/14.007g= 1.0708 N/0.8325 = 1.2862 x 4 = 5.1448 mole

20g of O/1 x 1mole/15,9994g= 1.2500 O/0.8325 = 1.5015 x 4 = 6.006 mole

The Molecular formula and Empirical formula would be C4N5O4.

Another example:

http://www.chem.tamu.edu/class/majors/tutorialnotefiles/empirical.htm

That is how percent composition, Empirical formulas, and Molecular formulas are calculated.

Sources:

http://www.chem.tamu.edu/class/majors/tutorialnotefiles/empirical.htm

https://www.delsearegional.us/Academic/Classes/highschool/science/chemistry/firstyear/Notes/academic/Unit03/06percent.html

http://virtualgardner2.weebly.com/h-chemistry-unit-3.html

http://chemistry.about.com/od/chemistryglossary/g/Percent-Composition-Definition.htm

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