How do you balance the chemical equation naoh + h2so4 > na2so4 +h2o

Make sure there are the same number of atoms of each element on either side.
1) Check each one.
2) If one's out of balance, alter the equation to balance it and go back to 1)
3) When everything's balanced, you're finished!

We have 1 Na on the left and 2 Na on the right here. We'll need another NaOH to balance it:
2 NaOH + H₂SO₄ > Na₂SO₄ + H₂O
Now O is out of balance. There's 6 on the left and 5 on the right. We'll need more H₂O:
2 NaOH + H₂SO₄ > Na₂SO₄ + 2 H₂O
Fortunately H is in balance. S is also in balance here, so looks like we did it!

Need any more help?

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The mass has escaped into a gas which you can not see so the law stays true.