The basic formula for mass percent of a compound is: mass percent = (molar mass of element/total molecular mass of compound) x 100. The molar mass of a compound is the amount of mass for one mole of an element while the molecular mass is the amount of mass for one mole of the entire compound. You must multiply by 100 at the end to express the value as a percentage.  Write out the equation at the beginning of every problem: mass percent = (molar mass of element/total molecular mass of compound) x 100. Both values have units of grams per mole (g/mol). This means the units will cancel each other out when you solve the equation. When you aren’t given masses, you can find the mass percent of an element within a compound using molar mass. Example 1: Find the mass percent of Hydrogen in a water molecule.  Example 2: Find the mass percent of carbon in a glucose molecule. If you are not given the chemical formulas for each compound, you will need to write them out. If you are given the chemical formulas you may skip this step, and proceed to the "Find the mass of each element" step.  Example 1: Write out the chemical formula for water, H2O. Example 2: Write out the chemical formula for glucose, C6H12O6. Look up the molecular weight of each element in your chemical formulas on the periodic table. The mass of an element can usually be found underneath the chemical symbol. Write down the masses of every element in the compound.  Example 1: Look up the molecular weight of Oxygen, 15.9994; and the molecular weight of Hydrogen, 1.0079.  Example 2: Look up the molecular weight of Carbon, 12.0107; Oxygen, 15.9994; and Hydrogen, 1.0079. Identify how many moles (mole ratio) of each element are in your chemical compounds. The mole ratio is given by the subscript number in the compound. Multiply the molecular mass of each element by this mole ratio.  Example 1: Hydrogen has a subscript of two while oxygen has a subscript of 1. Therefore, multiply the molecular mass of Hydrogen by 2, 1.00794 X 2 = 2.01588; and leave the molecular mass of Oxygen as is, 15.9994 (multiplied by one). Example 2: Carbon has a subscript of 6, hydrogen, 12, and oxygen, 6. Multiplying each element by its subscript gives you:  Carbon (12.0107*6) = 72.0642 Hydrogen (1.00794*12) = 12.09528 Oxygen (15.9994*6) = 95.9964 Add up the total mass of all the elements in your compounds. Using the masses calculated using the mole ratio, you can calculate the total mass of the compound. This number will be the denominator of the mass percent equation.  Example 1: Add 2.01588 g/mol (the mass of two moles of Hydrogen atoms) with 15.9994 g/mol (the mass of a single mole of Oxygen atoms) and get 18.01528 g/mol. Example 2: Add all of the calculated molar masses together: Carbon + Hydrogen + Oxygen = 72.0642 + 12.09528 + 95.9964 = 180.156 g/mol. When asked to find the "mass percent", you are being asked to find the mass of a particular element in a compound, as a percentage of the total mass of all elements. Identify the mass of the element-in-question and write it down. The mass is the mass calculated using the mole ratio. This number is the numerator in the mass percent equation.  Example 1: The mass of hydrogen in the compound is 2.01588 g/mol (the mass of two moles of hydrogen atoms). Example 2: The mass of carbon in the compound is 72.0642 g/mol (the mass of six moles of carbon atoms). Once you have determined the values for each variable, plug them into the equation defined in the first step: mass percent = (molar mass of the element/total molecular mass of compound) x 100.  Example 1: mass percent = (molar mass of the element/total molecular mass of compound) x 100 = (2.01588/18.01528) x 100. Example 2: mass percent = (molar mass of the element/total molecular mass of compound) x 100 = (72.0642/180.156) x 100. Now that the equation is filled in, simply solve to calculate the mass percent.  Divide the mass of the element by the total mass of the compound and multiply by 100. This will give you the mass percent of the element.  Example 1: mass percent = (2.01588/18.01528) x 100 = 0.11189 x 100 = 11.18%. Thus, the mass percent of Hydrogen atoms in a water molecule is 11.18%. Example 2: mass percent = (molar mass of the element/total molecular mass of compound) x 100 = (72.0642/180.156) x 100 = 0.4000 x 100 = 40.00%. Thus, the mass percent of carbon atoms in a molecule of glucose is 40.00%.
حدد قانون حساب نسبة الكتلة لمركب كيميائي. اكتب الصيغة الكيميائية. احسب كتلة كل عنصر في المركب. اضرب الكتل في النسبة المولية. احسب الكتلة الكلية للمركب. حدد كتلة العنصر المطلوب موضع السؤال. عوّض عن المتغيرات في قانون حساب نسبة الكتلة. احسب نسبة الكتلة.