A spring scale measures weight. some even use them to measure the weight of fish
The coefficients of the substances give you the ratio of the number of moles.
The 4 before the H2O and the 5 before the HCl tell you that for every 5 moles of HCl created, 4 moles of H2O had to react.
Therefore:

can be used to find the moles of H2O that react.
Oxygen (O), Sulphur (S) and Selenium (Se) have similar chemical properties because they all have the same number of electrons in their outermost shells.
Atoms with the same number of electrons in their outermost shells can be found in the same group.
A group consists of elements arranged in the same column (from top to bottom) on the periodic table. Elements on the same group have similar chemical properties.
Oxygen (O), Sulphur (S) and Selenium (Se) are in group eight. All three elements are highly electronegative and have 6 electrons in their outermost shell. They also carry ab charge of -2.
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